IRC is starting to echo a fairly common theme lately, that of games that have had horrible off-putting demos but turn out to be pretty good (or excellent). From my personal top-ten this year, four games were cited by either myself or others to have very bad demos but later turned out to be loved: Catherine, El Shaddai: Ascension of the Metatron, Majin and the Forsaken Kingdom, and Resonance of Fate. Even Batman: Arkham Asylum, a game almost universally loved, had a demo that received a very tepid (or downright bad) response.
I personally don't play as many demos as some, but fishing back through my memory I can't think of a single game in recent years that I wasn't sure about, then played the demo, then said to myself "Hells yeah! Day one purchase, baybeee." Unfortunately, I've played several demos that have soured me on the game, yet I still picked them up and loved them later.
So here's a question for the goodjer collective: Do you think demos help games? Are they a necessary evil? Have you personally bought a game recently specifically because of the demo? If so, why? Have you avoided games because of the demo that you later really enjoyed?