Bought it it yesterday and I am VERY pleased by the difficultly, the physics and the presenetion.
It is REALLY hard. It has 50 levels but it gets HARD after the first 10, however, it is the FUN kind of hard. You have to think, to tinker and to try different options. That's what's great about physics games.
It is essentially the strangest combo between "Bridge Builder" and "Gish". It is funny, slick, with pretty graphics and great sound but in the end, you essentially build structures with different materials.
It is not a game for everyone, but just the kind I'd suggest for anyone to try because you just might love it.
All in all, it is an awesome game and well worth the 20$.