For the most part, I trust demos. If I didn't enjoy a demo, I probably won't pick up the game. But there are a few exceptions.
For instance, Hitman Blood Money had a terrible demo. It was pretty much a giant tutorial level, and that was it. But I gave it a shot anyway. And boy, am I glad I did. Demos should never only be the tutorial, no matter how long it is.
On the other hand, some demos can actually be better than the games themselves. For example, Dark Messiah gave you an awesome area with a ton of ways to waste baddies in all kinds of creative ways. The full game was just more of that, over and over, with some scenery changes. Only with some godawful zombie fighting bits every now and then. The progression of the story almost gave you less incentive to be creative with your playstyle.