I tried this out. Really fun. A good combination of an intellectual challenge regarding how the physics will work, and the satisfaction of seeing it work on the screen.
After I got through the 21 levels, I went back and tried challenges for myself like how few pieces could I use complete each level, or tried to get the trajectory of the ball to be as smooth as possible straight to the target with no bounces, etc.
I'm surprised there isn't more discussion of it here-- in the iPhone world it seems to be all the rage.