Just Cause 2 and Deus Ex 3 demos sold me pretty hard on those games, and the Duke Nukem demo made it an easy decision not to buy the game. Some demos are great for making judgments, some less so, but I think overall they are pretty limited so I would say previews are equal or better almost every time.
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_foolmo_
'so I can potentially win the guru without getting the final right?' - BlAcK TuRtLe