I have found games where there is an awesome demo and the actual game really sucks. However, for the most part if the demo shows a promising game, I'll buy it. If the demo proves that the game sucks, I don't waste my money. So, could demos adversely affect game sales? Yes...when the demo shows that the game sucks. (I know it was already said, but it is worth repeating.) If the demo show that the game is a kick-ass game, it will help sales. I still think that hype and word-of-mouth are what really drive game sales.