I downloaded the demo of it yesterday. I'd have to say i'm hooked on this game. However, i'd like to see what buying it give me over the demo.
The retail version gives you more places to go, more tools to help you get there, and more to do when you arrive. The demo only has two worlds, Kerbin and Mun, while the retail version was just updated and now includes 5 planets and 8 moons to explore. The retail version has about 5 times as many parts as in the demo, including larger rockets, landing gear, and jet engines so that you can design space planes. The retail version also includes the ability to control the rocket from the cockpit, and go on spacewalks with your crew (which is a lot of fun when combining jet packs and low gravity). There's also a bunch of graphical improvements and bug-fixes and physics tweaking. Eventually, they plan to include a career mode where you'll have to worry about budgets and limited parts, as well as an expanded solar system (they're planning more planets as well as asteroids, comets, rings, and so on), and docking.
You definitely have a problem when Jebediah starts freaking out.
Here's some pics from my first trip out to Duna (the Mars stand-in).
This is my ship. It wobbles like crazy getting into orbit, but it just about manages.
Here it is in orbit of Kerbin.
The view from inside the capsule as the sun sets.
Kerbin and Mun in the distance as I head out into interplanetary space.
113 days after lift-off, I finally approach Duna and its moon, Ike.
In Duna orbit.
Jebediah Kerman becomes the first Kerbal to set foot on Duna. Here's hoping he has enough fuel to get back.
And here's a bonus picture of my first successful landing on the tiny ice moon Minmus.