Strangely addictive.
I do love the physics in the game. But there is room for improvement.
What I'd do (now this is just me, here) is update it a bit and add some costumes of people other people really dislike. *ahem* Paris Hilton *ahem* Some custom backgrounds or weapons would be nice. That way, people won't get bored with it really quickly.
Or maybe there could be some kind of finishing move. Press a certain button after so many hits and WHAM!!!! A giant fist smashes the little dude's head in!
Just my suggestions. Do whatever you want with it.