Did you watch Schell Games CEO Jesse Schell's D.I.C.E. presentation last week. Very interesting stuff, right? The most interesting bit comes about 10 minutes into it, where he points out some interesting data on game demos and how they can actually hurt game sales.
You'd think that letting people try out your game would help sell it, but it turns out that the games that sell the most copies have built up the expectations of gamers. Giving them a demo takes away from that expectation. In other words, now that they've tried it, they really don't feel like they need to go out and get it anymore. EEDAR's data of Xbox 360 cumulative sales shows that games with a demo and trailer sold half as much as ones with just a trailer alone.
Schell says that teasing with a trailer is the best way to get people to want to try your game. Cut them off there, he says, and avoid the demo. He says that the risk with releasing a demo is that people will try your game, but that's all you'll get out of them.
I'm sure I'm one of a very small number of gamers on this, but I'll only download demos of games I never intend to buy. I don't want to play an inferior snippet of a game I expect to play fully later. Of course, the downside to this approach is that I could end up spending money on something that I won't want to play.
Do demos make you want to buy a game?