thinking back over time, demos tend to have a weird affect, they alot of times have killed games for me, like Star Wars Unleashed, it was a good demo, but playing it made me feel like I was afraid of, that it would be sort of a hack and slash game hallway after hallway of slaying guys, and that kind of turned me off, same with Too human, so here I was on the fence, even a little more on the positive side and the demos pushed to the negative camp. But they've also done positive things, I had the battlefield bad company demo early and loved it and even showed it off to a ton of friends who ended up picking the game up, so I actually helped sell it to other people in that case.
There is another negative potential, the kind were the demo is good, but gives you just enough of the experience to fill you up, where your like I loved the demo, and even go back and play it alot, but after a few demo hours it seemed unlikely the game had enough left to offer me for a full priced purchase. I bought Burnout paradise but I could actually see their demo having that affect. Even Bad Company's demo could do that. being that they both offer an online experience that could give you countless hours of gameplay.