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