I wrote a blog a while back about the dishonesty of Yelp. In many cases their salespeople pressure small businesses by offering the opportunity to host favorable reviews at the top of the page and threatening to do the same with bad reviews if the small business doesn't buy Yelp advertising.
My blog goes on for pages and pages of comments supporting the experience of many small businesses.
Yelp is now five years old, has expanded to the entire United States and is still running a dishonest operation.
This should let you know two things. First there is a desperate need for a reliable online reviewing mechanism. Second word-of-mouth about a dishonest Yelp is not effective so far.