Interesting article here on those customer-contributed reviews at Amazon.com.

Obviously, people write reviews because one way or another they get worked-up about some book. But it seems to me people are far more likely to write positive reviews; at any rate, I really have to dislike something a lot before I will bother to agitate against it. In practice then, it is the bad reviews that are generally more useful. If the bad reviews are generally of the inane, "this sucks" variety, that generally augurs well for the book; and a thoughtful vat of bile offers far more insight than several dozen sugary declarations of enthusiasm.

In fact, instead of platitudinous blurbs, publishers should solicit unfavourable comments and print those on dust jackets. This will also put an end to the corrupt mutual back-scratching that exists, believe it or not, between blurbers & blurbees.