There was an interesting article in The New York Times by David Streitfeld talking about the new business of selling 5 star reviews.
Customer reviews are important as marketing tools, so the fine line between criticism and marketing has become fuzzy. Given the power of these reviews to drive business, it was inevitable that this would happen. Hell, I do it when I look for one of 500 different brands of headphones. Doesn’t everybody? So it makes a lot of business sense to amass these 5 star reviews (or I suppose one star hit pieces for your competitors). Check out Fiverr, for instance.
There is a 90 page academic study coming out of Cornell that analyzes the system at Amazon.com and comes up finding it wanting.
Of course, the objection is that reviewers for the serious media are paid very well, indeed. The difference is that they are paid to tell the truth about their experience of reading the books, whether they choose to give it a 5 star rating or no stars at all.