Kerbal Space Program is a great game for those who enjoy space and some of the challenges that come with spacetravel. This is where you get to design your own rockets, spaceplanes, rovers or anything your imagination can create with parts provided and fly them, well most of them crash and explode if you are new to the game. But in this game failure is almost just as fun as success because when you get your crazy contraption to actually work is amazingly rewarding. I've logged over 700 hours and I'm still challanged.
The modding community is very active and there are lot's of great mods out there if you ever need more to do.
Although Kerbal Space Program is a perfect 10/10 in my book because i love the struggle to get my rockets to work, I give it an 8.
That brings me to the con: For the casual gamer that wants to explore space quick and easy, without a lot of work and planning will probably find this game "too much work" and since the game is still early access, the tutorial is a bit outdated. You can expect a few bugs and some prefomance issues on older computers.