I've never bought one, but considered it from time to time. The two big problems with it are (1) many of the included attractions aren't that great or are free anyway and (2) in order to get your money's worth, you have to run around like a nut trying to squeeze in as many attractions as possible.
Personally, I prefer a much more leisurely vacation. At anything less than a frantic pace, the powerpass isn't a deal.