It's a great game for multiplayer. Don't just play the demo, there's much more to it.

User Rating: 8.1 | Small Arms X360
I'll summarize why I think it is a good game, deserving of a pretty decent score. Gameplay: It's quite simple, not a lot of depth, and the camera can get annoying as it tends to lose track of your character, but it's also very intuitive and can get very fun and hectic. One problem is that your character seems to run out of ammo quite fast, which can be a bit annoying at first. But after a bit of playing, you realize this actually makes the game even funner, as you have to be agressive going after ammo. Some of the best fun is to be had when you have no ammo and all you can do is hit opponents with your weapon as you race for the ammo. Still, 33% more ammo might have been better.

Graphics: Simply amazing for an XBLA game. No doubt about it, this is the most technically impressive game. Saying this game looks bad as an XBLA game is like saying GOW looks bad as a 360 game. Sound: OK, nothing special, not bad.
Story: Not really much here, but i'm not sure it's really needed either.
The characters are pretty cool. And i think the weapon balance is quite well done. Some weapons work better on different stages, so practice and strategy really do matter. Experimenting with weapons can pay off.

Overall it's pretty good, really good for an XBLA game. It's well worth the price, especially for the multiplayer, which is much better than the single player experience. Easily one of the best XBLA games out. Yes, it's a Smash Bros. clone, but it's a pretty darn good one. And for a cheap price!!

(Apparently Gamespot didn't like my first review because I criticized Jeff G's review. Suffice it to say that I disagree with his review, which apparently I cannot write about or give reasons, by GS's TOS)