Yeah, I always prefer a game to have a demo than not have one, a demo will really encourage me to a buy a game I am not sure about. For example, I'm curious about Dragons Crown (dubious art style aside) but I'm not willing to spend £30 on a game I know nothing about. On the other side of the coin, I love the game Puppeteer, a game I knew nothing about and had no interest in before I played the demo.
Demo's can also be misleading though, I brought Brutal Legend based on the demo of it being a crazy hack 'n' slash/car driving game. The rest of the game however was very disappointing. But again, I hated the Batman: Arkham Asylum demo, that really put me off the game. The only reason I picked it up is because it got fantastic reviews from everyone, so I took a chance and ended up loving it.