Just awesome :)
Fun and more adictive than you may think.
Nice toy! The controls are very simple and perhaps not user friendly. But it is fun to play with gravity for some time. From the point of learning some physics one has to say that it is not more than a nice to play with tool. But playing is important for learning. So over all I like it!
It's a fun program to play around with, but the tools aren't great for a solid understanding of the mechanics behind what you're doing - or they're not completely clear on them. Anyhow it's very fun to try and build star systems with it, you can get some nice and/or complex orbits going!