It's rare, but sometimes a demo can put a game that was completely off my radar onto the buy list.
Enslaved was the most recent. Knew little about it other than being developed by Ninja Theory who I associate with Heavenly Sword (another game that I enjoyed and was sold on after playing the demo). Hopefully the entire game keeps the same pace as that demo had a big grin on my face for being just plain fun.
Heavy Rain made me want to play the full game. Infamous really sold me on the game. But I think it's the more obscure games that stand more to gain by making a demo if they don't have much of a budget for marketing and advertising.
But with all the good games available and or soon to be released, I typically use demos to rule out games that I won't buy rather than find more games that I want to buy/play. I have way too many unfinished games that are fully worth finishing as is.
And a lot of times, a demo that was bad or just let me know that the game wasn't what I thought/hoped it would be saves me the cost of a disappointing purchase.
There are quite a few games in my collection that I wouldn't have bought had I played a demo first.
