I agree for the most part... the demo GOW3, left me unimpressed so when my brother rented it I was still playing another game, and didn't bother renting it again.
I played uncharted in the store... and it was underwhelming. Now I'm not the biggest uncharted fan in the world... but I wouldn't have waited a whole year to play it, if I knew it was gonna be as good as it was.
Mafia II... Demo was bad didn't buy.
Just Cause... Didn't like it, didn't buy/rent.
Ditto with Kane and Lynch II
Valkyria Chronicles... the demo was really boring... I almost bought it, but the demo turned me off... I finally was out of games to play and wanted an HD game so I gave this a shot... and it was great.
FFXII demo... didn't like the demo, but I bought it anyways because it's final fantasy.
Maybe some of these games were good, and maybe some of them sucked.
Now there is one game that I wouldn't have boughten, but I bought because I absolutely loved the game
Lara Croft and the Guardian of Light.
Due to the negative perception of the tomb raider series... I debated ever really to try the demo. The demo was awesome in my opinion. And I bought it and loved the game as well.
So basically... demo's help me not buy games I might normally buy/rent. I'm happy cause it saves me money, but atleast for me personally, demo's have turned me away from a lot more games than convinced me to buy.
Now there's some franchises such as Final Fantasy (the mainline ones) that I'll buy no matter what... (though actually I bought the JP advent children so I could have the blu-ray movie with the FFXIII JP demo). As well as WRPG bioware games... I'll basically no matter what as well, no matter if the demo's are good or not.