Have you ever really wanted a game, played the demo, and then gone "eww no thanks"?  Ever played a demo you were casually interested in and then gone "Holy balls that was awesome I need that now!"?  I bring this up because last night:

I played the demo for Resistance 3.  I had played 1 and 2 and enjoyed them but had completely written off R3 as a "meh" game.  I finally got bored last night and decided to fire it up and holy shit I was blown away! 

It was actually beautiful,  the shooting and aiming is soooo tight and good, and the battles felt like they were really organic instead of "get from point A to B".  I was floored by how good it looked, not because of the textures (there were some low res ones) but the particle affects and the amount of movement on screen is phenomenal!  I might buy it now.

Any similar (or opposite) experiences? 

Turok 2 (played the PC demo, bought for N64...), Heroes 3 and Mirror's Edge are the only games I can remember buying because of their demos.
The demos for Killzone 2 and Heavy Rain really turned me off but I wasn't that interested in those titles to begin with.

Yeah I'm sure there's been some, but its hard to think of any examples off of the top of my head. One I can think of is Haze, I was super pumped for a good 4 player splitscreen game plus it was early on in the PS3's life when I was desperate for good games...and of course it was shit, the demo plus all the horrid reviews turned me off.

Most of the time I just play demos for games that I figure will be write offs so most of the time the demo just confirms that it is indeed a write off. If its a game I know will be good I don't even bother with the demo really.

Most of the time!
Demo's are an important asset of today's gaming systems if you ask me (not that Wii is particularly strong in that).
The thing with demos is that developers do not seem to always understand what they should be offering in them, if an initial level, if a specially design level, if the multiplayer experience, etc.

I bought The Lord of the Rings: Conquest because of the demo. I was not disappointed.

I waited 2 years before buying Uncharted: Drake's Fortune because of the boring demo. It ended up being an amazing game.

Left 4 Dead, on X360 (bought)

Dead Rising 2, Burnout Paradise, Mass Effect 2 demos, stopped me from buying the games

Games I purchased because of their demo
- Warhawk
- Grid
- Battlefield 1943

Games I skipped because of the demo
- Bulletstorm
- Enslaved

SAME!!!!

I bought Player Manager 2000, Time Splitters 2 amongst others simply because of their demos and they are now part of my fave 50 of all time.

Bioshock.

Age of Empires. The first one. I played that demo dozens of times, and it's still one of the games I've spent more time with.