I can't remember when a demo made me buy a game the last time, when I initially didn't already plan to purchase it. Kingdoms of Amalur might have been the last candidate, though I didn't end up buying that game.
I generally don't play that many demos anymore these days. Those are imho relicts from an age before the internet. When you only had 1-3 gaming magazines and no other sources. Today we have instant access to tons of reviews and opinions, not to mention all those videos. Add on top the experience of some dozens years of gaming and ... uh, whatfor would I really really need a demo?