Yes Demos have made a difference for me.
DEMOES THAT MADE ME BUY:
The Uncharted Drake's Fortune Demo was boring and made me not buy it until about a year later when word of mouth got around saying Drake's Fortune was a solid game. Guess the word of mouth was correct.
The MGS4 Demo was just orgasmic :D! Around that time I was so hyped for it and getting to play the demo back then was like a dream come true. It was fantastic and I could not wait for MGS4.
Killzone 2 demo was a turn on. It got me used to the weighty feel and controls for the real game and it was good fun.
LittleBigPlanet demo persuaded me to buy the game. It was a must buy after playing through the demo. The demo itself was very well put together and who knows, if I never played that demo, I would've never had picked up LBP and definitely not be interested in LBP 2.
Battlefield Bad Company 2 Multiplayer Demo was a blast. It got me hooked up with the Battlefield franchise again and I eventually got it this month because in March I had God of War III, Final Fantasy XIII and Heavy Rain to play.
DEMOES THAT WERE GOOD BUT NEVE GOT TO BUY:
The inFamous demo demonstrated very well as to what players can expect in the final game. I was hooked into the demo but around the release of inFamous, I didn't have time to buy it and as we moved into 2010, it just got worse and didn't have the time.
Split Second Demo I found was cool. I found the gameplay was good and all but same reason as inFamous, just don't have the time this year to pick it up.
DEMOES THAT MADE ME NOT BUY:
The Quantum Theory Demo absolutely killed the chances of me buying the game. It's so horribly sluggish and no fun.
Ninja Gaiden Sigma 2 Demo also turned me off. Just didn't like it. I hoped that it could convert me into a NG fan but guess it didn't.
Heavenly Sword Demo was so short that I kinda got pissed off and didn't make me wanna buy the game. Not only that but I didn't feel as if the gameplay was that good.
These's are only a handful of demo's I've mentioned and just wanted to show how demo's do affect my decision on their purchases.
