Mrs G points out just how overt some people are about 'working' the review system at Amazon. There is a forum full of people just begging to shill for you at $5 a pop.
I particularly like this one. The poster offers to whore out his 5-star praise and ends "no erotica please". Because, y'know, he has standards.
On person offers to review as three different people. Another says, with no apparent irony: "Positive reviews will help your product credibility."
I know how "credible" I would judge any book carrying reviews by these rent-a-fans.