Well, I've put together enough of a ruleset for my Badass Space Marines game that I could run it at my local gaming convention over the weekend. I ran two games, and I pressured a friend to run another.

They all went great, I was pleased to see things fit together just like they should. Before I talk about some of the actual play stuff, I'll just mention the four main features that (I think) make the game cool:
1. Marine player combat rolls don't directly damage creatures, they give you permission to describe action-movie style stunts that kill the creatures. Stunts may not be repeated. Higher rolls just allow more stunts.
2. The GM has no prep. There are mission sheets with mad-libs style blanks to fill in. Anyone can set these, and the backstory of the situation gets generated on the fly in play.
3. Taking damage, and having the extras die, powers up the player characters (this, of course, is stolen directly from Scarlet Wake).
4. Every marine has a specialty they can use for a bonus every combat roll if they work it into one of the stunts they make. These vary from the workaday ("Close combat", "Flanking") to the extremely cinematic ("Pithy one-liner", "Screaming abuse at the enemy").

Additionally, character generation consists of picking one of the pre-generated marines from the squad. They are defined only by name, rank/job, number of hit points, a specialty and what weapons they have.

First game I ran, I pretty much had a dream group of players. I knew most of them from previous years at this con or other previous gaming, and I knew these guys would go for it. They loved the stunt system and went crazy with SF/action movie cliches as they wasted aliens left and right. One danger with the "anyone can fill in the backstory" that I had forseen was that someone being stupid could insert a fact that would mess up play for the others. I have written in that "if everyone else thinks it's lame, you can't do it" for these cases. Now, this first session got a few really stupid facts in there - specifically, the enemy creatures were hybrids of alien insectoid lifeforms and... Australians. But everyone found that funny, no need to invoke the lameness rule, and it was all good. I guess the scare-element suffered a little, but the thought of a half-insect with stereotyped Aussie features is not without aspects to fear.

We also had astonishingly good characterisation, given the sparse information given for the characters. The first section of the game is purely character interaction and setting the first facts in the backstory, specifically to enable the players to build a little personality on their four bits of knowledge about each marine. I could not have hoped for this to be picked up as well as it was. Within minutes we had a long-standing bitter history between the sarge and a machinegunner, a clear feeling that nobody liked the lieutenant, and a bunch of other stuff that I can't recall but definitely was there - maybe I was distracted too much listening to the sarge chew out this marine for talking back.

The real stumbling block in this run was pacing and the threat level. As the game goes on the gamemaster gains points (Fuel) which can be used to attack the marines with swarms of creatures. This game, I realised about three quarters of the way through that I had so many that the marines were utterly doomed. I quickly changed tactics to ease up on the marines (there are a few ways to do this, such as attacking with more but smaller groups). This worked okay, but it entailed a rethought of the number of points that the gamemaster gets, and the rules for allocating damage. Currently, the gamemaster gets a damage total (die roll plus number of aliens) which is applied as damage to marines. Anything goes as long as the extras take more than the players (this was to force the 'extras die first' aspect of this kind of film). However, the player marines need to take damage to power up, so a lot of damage got thrown onto extras and failed to power up the player marines. The next revision will have the rule revised to say "If you have enough damage to kill any extra in the combat, you must kill one. After that, anything goes." This should both be better for the desired purpose and also leave the GM more freedom to injure player or extra marines to maintain game pacing and tension.

The second session was quite different. We had less initial characterisation. Also, one player was clearly quite thrown by the system. He played the lieutenant, and decided to be a coward. But the rules don't actually support this. The way I wrote them, you implicitly don't even have the option of not fighting the aliens (next revision this will be explicit, in fact). Then he said he wanted to commit suicide. This also isn't possible. The combat rules do not allow marines to kill anything but the enemy creatures. When this had all come out, the player seemed to suddenly get it. He made the lieutenant's combat roll and described his stunts as the character's failed suicide attempt knocking out some of the enemy (psycho robots this time). The robots then devoured the character and the player grabbed the sarge and spent the rest of the game kicking ass with the best of them. Oh yeah, and later on it turned out that the character was actually behind the whole thing, hadn't really died then and was eventually killed by space pirates because the other marines screwed up the deal he made... which was basically the coolest thing you could possibly do with that, if you ask me.

I'd made a small change to the GM Fuel rules after the morning's game. This partly contributed to the opposite effect - this time, the marines killed robots with wild abandon and I never seemed to be able to touch them (in a large sense - plenty of marines died, but it never looked like they would all be killed there). This wasn't just the result of a badly thought-out change, however, as I worked out the difference and it would not have been that great. What this group did better was setting the facts well. You can get a bonus on your combat roll by using one of the facts that has been written in by someone earlier, and they had put in some that were really useful for bonuses. The main one was that they were in a mining base that specialised in a highly explosive mineral - plenty of robots went down due to exploding minerals in the course of that game.

The other thing that happened in the second game was "stunt fatigue": people had to come up with so many unique combat moves, that they began to run out of ideas. Once someone mentioned this, a few of us began making a real effort to help each other out with suggestions whenever a player got stumped. This worked well, and really got a wonderful group vibe going. We had situations where the active player would look confused, someone would make a suggestion, then three or four other people would add cool elaborations on top of that suggestion. Then the active player would grin and say "yeah, I do that". Awesome stuff, that. And, in the end, we always got a load of really neat stunts that felt exactly like the kind of things you get in action movies. We had pipes venting freezing fluids, smashing things against walls, lava rivers, ricochets, accidental alien kills, and so on.

We also got a great one-up-manship contest going on after one player killed maybe 7 robots in one turn. I mentioned that the "high score" was 12 from the first playtest game. Four of the players then decided to go for broke in an attempt to top this, leading to some great chat between the characters ("This is how we do it in breeding vat B" stands out from the rest of the smack talking) and finally to someone nailing 14 robots and securing the high score. I've decided that I'll keep track of the high score on the game's website, hopefully to encourage the same kind of play in others.

Now, I had to leave before my friend finished running his session of the game so I haven't got the full rundown on how it went. We were sharing the same room at the con, so I could tell it was going well - plenty of shouting, laughing and phrases overheard left no doubt there. I got a look at the mission template too, and it looked like something generated by people enjoying themselves.

Hope the rest of you interested in and/or inspired by Descent find that of use. I think it's pretty amusing that I could take the ideas in that original Descent thread and build such a different game from it. The way that wanting basically the same cool stuff happening could be reached via such different directions is really... well, something... But it really seems like that is what happened.

Mike, last night I fell asleep laughing at how bad that Lt. was.  Awesomely fun session, great combat engine that seems to do exactly what you intend it to do.  I did have a few thoughts after the game (points 1 and 6 are probably the most relevant, but point 4 was a key and side for me and to "what is your game about?"):

1.  Duration. The session felt slightly long to me.  I wondered whether this was because there were an equal number of Facts to be filled out on each phase.  Perhaps if there were slightly less Facts in each successive phase, the sense of pacing would increase as we drove towards the end.

2.  Inconsistent number of Facts?  We had to fill out all of the Facts in each of phases 1, 2 and 3 before we could move to the next phase.  For those who don't know, each phase describes in act of the story: Shipping out, Arrival at the Hot Zone, the Enemy Attacks and Game Over.  However in phase 4, it seemed like we only had to fill out two Facts in order to trigger the endgame.  This actually threw me for a couple of seconds - the logic of it seemed unclear.

3.  Character interactions. This may have been my own fault, but I felt like I didn't interact with some of the players of the other marines as much as I could have.  I set up a rivalry with Gino's character, reinforced it with a Fact that he had shot me in the back (twice!) and left it at that.  It struck me that phases 1 and 2 could be well served by having more marine versus marine character interactions (to draw on in combat).

4.  Facts.  Players need to realise that Facts need to be well selected in order to give them bonuses in combat.  Everything in the game bends (or should bend?) towards making the stunt system work effectively.

5.  Facts 2.  Contributing Facts gives you Fuel, which give you bonuses in combat.  Great.  But initially I didn't know/didn't realise/completely forgot that you also earned Fuel via PC death.  I felt that Fuel was a scarce resource, and that made me hesitant to contribute more than my 'fair share' of Facts.  So I guess there are two things here: a) no system for arbitrating which player gets to contribute Facts, so GM fiat tended to come into play; and b) the benefits of having your player die need to be explained more clearly, perhaps.

6.  Adversity.  I think you'll agree that by the end of our session there was less tension about whether our characters would survive and more player enjoyment at exploiting the combat system.  And I think that tension needs to be in there.  However, I think a smart informed group playing Badass Space Marines needs to be able to make smart informed choices about how to maximise their chances of survival - while at the same time the GM should have a fair chance of killing them.  Obviously, you mentioned redesigns in your initial post.  The thing is I can't really judge this element of the game because I didn't know how the 'rules' for how much fuel the GM earned, how it was taken away from the GM & how the GM allocated fuel to each attack by the aliens.

7.  Improving weapons.  I didn't see much point in spending Fuel to do this, given that I expected my Marine to die at any time.  The cost of improving a weapon by one point seemed to be outweighed by the benefits of using fuel in a combat then-and-there.

8.  Keeping track of names.  I noticed how Leslie folded a piece of paper and wrote Evans on that so that we could tell what her Marine was called.  Things like that would be a good idea for all the character handouts; one side has the Stats, the other side has the name in big font.  You fold it over so the player can read the stats and everyone else can read the name.

9.  Supporting characters.  Maybe players should just grab unfolded Marines from the marine pool, so you can actually see what their bonuses are.

10. Replayability.  I wondered whether having fact sheets with different facts on them would enhance the replayability of the game, or whether you would actually get a different experience if you just stuck with filling out the fact sheet that you've provided.  What was your experience with this?

11.  Genre emulation.  Kind of related to the above - I'm not sure whether this game is a full-on rerun of 'Aliens' or its own thing entirely (certainly I didn't expect the amount of low gravity space wrestling with killer robots that we had in our game) but I had too much fun to consider this a major issue.  Thought I'd bring it up though.

In summary, cool game.

Thanks for the comments, Steve.

There's a lot there for me to digest but it all looks useful - I like the foldable character name idea especially. I had already decided to make the character slips bigger, but I think I'll make them be a name sign as well.

I'll think over the rest of it and probably start a thread in game design to discuss that...

Urk, yeah there's a lot to digest. I didn't realise this was in Actual Play, not Indie Game Design. Here's a less bullet-pointy, more gestalty summary:

The stunt system is great at encouraging players to get into Director mode. I could actually see players getting more comfortable at inserting first small objects (like pipes and airlocks) then larger and larger things (APCs, mechas, overhead cranes) into the environment. Filling out the Fact sheet really primes you for getting into this.

Quite brilliant, really.

The game perfectly captures the see-saw between machismo and outright terror. Every time I described a stunt I felt super-cool & I also loved egging on everyone else's stunts - so you're right, we had a great 'we're a platoon' vibe going on ... but then Mike, you'd spend fuel, introduce another 50 aliens & we'd be stuffed all over again, immediately beginning to freak out.

I also loved how characterisation emerges from just the smallest details on the character sheet (which is basically a name, a rank, a speciality - 'oneliners', 'flanking', 'right place, right time', and your weapons - actually I'd love to see some more variety in the weapons, too). And rapid PC turnover created this really interesting web of new character relationships.