overall, a great game
Challenge mode reminds me of mission mode from the soul caliber series, which I thought was a good idea. There wasn't many interesting variations though, except that one challenge level where you had to fight as salad fingers in the dark. I got really pissed playing that level. It wasn't fun at all, just annoying. Salad fingers is shit, more like Edward Shit Fingers amirite?
Just an idea: you could have put in a side-scrolling platform mode without much effort I'd imagine, and that would have added a lot more play value to the game.