Normally I don't buy newer games without trying a demo first (of course I'll get D2:ED in any case). With older (cheaper) games I am willing to gamble if I've seen positive reviews from trusted sources.
However, most game purchase decisions are made in the store looking at boxes, with a smaller chunk being based on reviews or advertisements. IIRC from some study published awhile ago, a fairly small portion of gamers base their purchases on demos or playing the game (ie at a friend's place or store display).
