I've been playing KSP for over a year now (since 0.8x). The best part about it is that there are NO RAILS in the game. You build your ship, you launch it, it flies. If you screw up, it wrecks. There's no fake scripted shit going on in the game. Though I believe the physics are not 100% accurate (for example, it won't do Lagrange points in orbit) they are close enough for most of the things you'd want to do, and the universe is scaled-down so that missions don't actually take months. You do have time-warp to wait out those year-long coasting phases.
The latest version is so feature-rich that I've been struggling to even try each of the new features. You can dock, you can go EVA, you can fly NERVA rockets and ion engines (though they are far more powerful than the real thing as I learned about reading after the Dawn probe).
Basically, KSP inspired me to go and look up EVERY planetary probe and mission from the last half-century. I have become far more aware of the amazing accomplishments of NASA (and the Soviets too) and I think the only real-world mission you can't do right now with it would be Curiosity, but some modder out there is probably working on it.
It is a FANTASTIC tool for education. After playing it for several months I've come to realize how silly all the Star Wars-type dogfights are, how utterly impossible and impractical they would be in a real world environment, and why it is that we don't just have interstellar travel 'cause we wanna. And I've spent a ton of time honing my math skills just to get a feeble grasp of what happens and why.