I've been wanting to play KSP, it looks absolutely fantastic and a very rewarding game. It does look like it has quite a learning curve. So much to do, so many ways to do it.
KSP reminds of Tekkit mod for Minecraft.
There is a big learning curve, but personally, myself with absolutely no prior training in anything close to aerospace engineering being able to pick up the game and have fun, that's saying something. There are 4 tutorials covering the basics of launching, finding your orbit, manouevering, going on an EVA and landing, all of which I HIGHLY reccomend if you want to play the game a little more "properly"
It is very much a sandbox game. You set goals for yourself, you learn at your own pace. at my skill level, having played for several months off and on, I can more often than not make a mun landing and back with the ships I design, but the game comes with stock ships too so you can get some ideas or flight experience from that. For me the most fun is building and flying your own ships. If you want to make a ship with 100 solid rocket boosters, do it. It won't fly very well but you can do it.
Incredibly, incredibly fun game. I would reccomend buying it from the KSP store so Squad gets a few extra bucks (I think steam takes a 20 or 25% cut of the sale price, a dev from another game told me once.) If you want to see videos of somebody who is basically, in my opinion, the defacto master of KSP, check out Scott Manley's channel on YouTube. Some other, more fun channels you should check out are KSP Insane Rocket Division and the test pilot series from HOCgaming.
http://www.youtube.com/user/szyzyg (Scott Manley)
(IRD)
http://www.youtube.com/show/ksptestpilot (KSP Test Pilot)
If you get into it, this is the sort of game that can eat hours and hours of your life and prove an incredible bang for your buck.
Back on the topic of Hitman, I DO like the built in "contracts" feature. Very smart. Adds replayability without actually needing any extra work involved, and challenges you to solve the missions in a way that you, or possibly even the devs, hadn't specifically thought of.