It's a brilliant engine. The game mechanics are excellent, and the gameplay is quite fun.
My only complaint is the lack of a "proper" story to the game. 10 missions go by quickly once you get focused, and the free mode is simply more of the same. There's simply no replay value.
If there is a future game planned (which I'd love) it should have a larger variety of gamemodes and gear. Online multiplayer would be harder to include, but it would be a nice touch.
Overall, this is a very good physics engine with quite a bit of potential. I'm impressed.