Depends what you like in a game. It's still in development (albeit quite far along), so things are still changing, particularly the career mode.
It's a game where you end up learning a lot about rocket science without even realising it. TWR, delta-V, Hohmann transfers, etc. etc. All whilst building frankly ridiculous rockets to transport little green men around an imaginary solar system. You can play it very seriously (try to accurately recreate the Apollo missions, for example), or just have fun (how stupid a contraption can I make fly, and how big an explosion can I make when it hits the ground?).