[quote name='jacknicklson']Not really, there are basically no rewards to getting everything[/quote]
Well, actually if you 100% a level, you unlock the theme song for that level. (basically you can select the song from a list and play it outside of the game). Some of the music is pretty good, but it still isn't really enough of an incentive (for me, anyway). Boss fights are really fun though, and you'd definitely want to replay those, just for the heck of it. I think the MSRP for this game should have been $30 to begin with though.