Misuse of anonymous review features is an ever-present danger to sites that offer that capability.
Review functions can be a good thing - they add content to a site (and specifically to a product page) and build community. Unfortunately, they are hard to police, particularly if they are anonymous or require minimal registration. People with a financial interest in the product can post positive or negative reviews, as can those with an axe to grind.
I don't think Amazon's "Did this review help" rating system is very effective right now.
If a community like WebmasterWorld allowed product reviews, one might use member status as a screen for posting them. If, say, a few hundred posts were required to post a review, drive-by type biased reviews would be impossible, and other members would have some idea of the level of expertise and the biases of the reviewer.
Despite the problems, though, I still read the reviews at Amazon if I'm looking at an unfamiliar product.