Personally, I was very disappointed at the Demo. It seems to suffer from the usual problem of restriction placed on demos of open-world games. Too often they are just a massive world locked into a tiny area of allowed play.
Don't get me wrong, the game looks amazing, and plays great. But the game's greatest feature - movement - is stilted when you can't go more than a couple hundred meters from your next objective.
What they should have done is a Just Cause 2 approach to the demo. Start the player in freeroam, and enclose a medium-sized vicinity to allow the player to explore and try a few missions.
Anyway, this is probably a buy for me. I'm torn between this and the fantastic LBP Vita. My cousin just got access to the beta, and it's truly amazing. Everything LBP2 did and more. Probably will get both, but it's hard to pass up on LBP's infinite replay value.