I don't really play demos that much. Ever since I played the Final Fantasy VIII demo on the PS1, they began to feel like a waste of time. I would play a demo for hours, a few times through, and then when I got the real game, I'd think "Damn. I've already played this part to death and I couldn't even save my data."
My most played demo would have to be Metal Gear Solid 2 on the PS2.
The last demo that I really played intensely would have to be Batman: Arkham Asylum on the PS3 and 360. I even played them side by side, flipping back and forth on the same television to see which one I liked the most.
I normally know if I'm going to buy a game well before there's a demo. If anything, a demo will turn me off to a game (Bayonetta) that I intended to buy because it throws too much information at you, too quickly. I prefer to download gameplay videos.
