To be honest, I think it's overrated.
What can I say? There's not anything particularly wrong with it; the graphics are fine, sound fits well, very interactive, a bit cliche (though it is a fighting game which I'm sure by now is impossible to make one non-cliche), and even a bit funny, especially with deaths like Pico's. The problem here is I simply did not have that much fun playing it. I found myself playing this game simply to unlock all the characters and levels and having very little fun. And even playing the game to unlock everything doesn't take long; I had every level and character unlocked (and the story for each character beaten) shortly after the game came out, and I haven't touched it since. It offers little replayability, but this is no fault of yours- playing fighting games against the CPU is no fun, and playing against humans is awkward on a single keyboard (this is a flaw in all computer fighting games).
Basically, it's great if you have a little time to kill, but there isn't much to do after the first hour or so of gameplay.
Don't get me wrong- this is a good game, and I think it definitely deserves to be in the top ten. I just think it is very overrated,