Toast_burner: keep in mind that the demo is very out dated.
Telika: I was really trying the demo as a demo. It's quite playable. And while I understand the amount of effort and detail put in it, I had the impression that the core elements were there, and that if I feel I've "exhausted" the demo (in terms of fun I take from the gameplay), then the additional devices of the full version (planets, rocket parts, walks) wouldn't add a huge lot to it.
Am I wrong, though ? Does the full version really add new dimensions to the gameplay ? They have added docking, so you can build spacecraft in orbit, go for an Apollo style Mun Landing, build space stations, bases, etc. Landing struts (essential), IVA (Intra Vehicular activities. Right now it's just an inside view, but it was added last update, .17, and .18 mainly focused on Docking, re texturing, etc.), tutorials (only two, but it was this update, .18.), probes, and scenarios. Right now it's in Alpha, but the full version is waaaaaayyy better than the demo. And if you by it now, all updates are FREE.