It's perfect for children, but not aimed too young so it can appeal to adults.
It involves a physics engine, and if all the various Apple Apps are to go by we LOVE to play around with a simple yet realistic physics engine, I think it offers just enough randomness to keep you looking for something 'cool' to happen.
The sounds are perfect, the graphics are perfect. And the objective is nicely in-between puzzle and action to grab all minds.
But it might also have been somewhat luck, I mean if one game or app get's to a certain level of success it kind of feeds back into it's self, because the various search engines and ranking systems work on popularity.