A must have for space/rocket fans!
KSP provides a large set of components and functionality with a relatively bug-free simulation of physics.
I've read reviews mentioning repetitive crashes or the lack of Steam Workshop integration, I personally had 2 KSP crashes in all my game time (77 hours) so that is fairly alright. The mod installation is as simple as putting the mod folder into your GameData folder, that's it.
Talking about mods, the mod support seems to be great. I've never looked into modding it myself, but since there are mods adding mineable ressources to planetoids or doing complete mapping scans from certain orbits there is a lot of room there :)
Who do I recommend this to?
"Engineers": People that like to Trial&Error in games, that like taking challenges and eventually mastering them. In KSP, the difficulty of these challenges depends on you (i.e. sending two rockets with station segments into orbit or try it in one turn, which is more cost efficient etc etc)
"Organizers": People that love doing stuff as calculating orbits (orbital period, "best suitable inclination" and so on), Thrust-To-Mass Ratios and all the tons of numbers you find in rocket science :P
"Pilots": People that have fun with coordinating a vessel in space near a fragile station to it's docking port ;)
Most of you will be all three of them, but interestingly you can lay your focus on whichever you want.
Also the humor of the game is amazing, so from my side:
A BIG YES to Kerbal Space Program!
(PS: If you decide to buy it now, check out Scott Manley's KSP tutorials. He's kind of the iconic figure of KSP tutorials and it will make your first hours a LOT more understandable)