Good reviews or bad reviews: which are more compelling?

At Wize, we spend a lot of time thinking about customer reviews and expert reviews, how they can be most beneficial to customers, and how businesses use them to drive traffic to their websites, stores, offices, etc. Generally, we agree that manufacturers and restaurant owners prefer it when they get good reviews. But what drives traffic to online retailers who sell products they don't manufacture? Are customers more compelled to read positive reviews or negative ones?

Our working theory, based on our own experience, is that bad reviews are a lot more compelling than good ones. For example, I know that most people love their iPods. But finding the person who hates theirs can be very revealing. Is the negative review good for Apple? Maybe not. But is it good for  Amazon.com?

We'd love to hear what you think. Do you tend to read more positive reviews or more negative ones? Why?