Demos do not represent final product. They should be called beta's.

Multiplayer demos are called beta's and we know that they are not final product.

The same think should be with single player demos.

Unless demo is out the same time as full game.

Vanquish's demo made me go from "Hrm, this could be good! I like Mikami" to "YEEEEESSSSSSSSSSS"

Infamous demo put me right off, they picked the worst/most boring part of the game to play.

But thankfully I borrowed the game of a mate and it was awesome.

A demo will only influence me if it's a game I'm unsure about. For example, I wasn't sure whether to buy Mafia 2, and the demo just completely killed my interest in it. I also bought Bioshock on the strength of the demo after being unsure.