Very good, if you actually take the time to play.
A lot of people were turned off within the first three levels; the first level is EXTREMELY hectic and confusing, and it's far too easy to die in it. Judging by a lot of the negative reviews, a lot of people stopped there and passed it off as bad. In example, MindChamber remarked that the characters were unmemorable; he obviously didn't play long enough to get the point that you were supposed to create your own using parts. Someone else said the characters were too similar; the same applies. I'm not talking about the characters so much as I am how little time these people apparently were willing to put into the game. I got the same impressions, but I kept playing. Once you get the hang of it, character appearances range more, and the levels flow a little more sensibly, it gets much better and far more enjoyable. The game is very cleanly put together and has an amazing variation in weapons and has extraordinary character customability. The combat gets hectic, but there is an art to fighting like that.
This game is very well made, and is extremely similar to Super Smash Bros, which it mimics very well; its downfall is that you made a piss poor choice with the first couple of levels. You really should have started players off with easier, more manageable fights.