Since this topic has popped back up, I'll just link to this story about "
Car critic quits over changed Chrysler 200 review". Even when the reviewer is being honest and not holding back, the people in charge of access to the review can weasel things around to make the product look better.
I've read threads on the Amazon discussion boards about people who've seen negative reviews of self-published books disappear after the author made a complaint to Amazon.