Crackdown. I couldn't buy it immediately, but when I did buy it. I played the shit out of that game.
On another note: From this point on I shall never play another demo again. I just play it and play it and play it, having no concrete source of income, I can't just go out and buy it. So usually the demos spoil the game for me. Like Crackdown for example, Overlord or UFC 2009. I'm not going to get that because I know I won't play it because I spoiled the game by overplaying the demo. The games that I have played the longest: Fable 2 and Rome: Total War, I never played any demos of them and I'm still playing one of them.