I remember my favorite demo ever was for Tony Hawk's Pro Skater 2. It was limited time (you could only play for two minutes before it made you start all over), and, with just one level available (and only two pieces of background music, which alternated each time you played the demo), limited features. But I played the hell out of that demo. I never actually bought the game, though I spent more time on that demo than I have on some full games that I've bought over the years. I think that getting a demo right is really just about giving the gamer a good taste of the game. That said, maybe there's something to be said (from the company's perspective) for not giving the gamer too much of a taste, after all, I never did move past that demo to the actual game.