When it comes to relying on the forthrightness and honesty of anonymous strangers for decisions like choosing a restaurant, nail salon or dry cleaner, Yelp can be a quick source for a quick review in a pinch. For much more serious decisions, however, like the hiring of a contractor to perform work in and around your home, you may want to consider going elsewhere for reliable information.
Because of it's open format and absence of requirement for reviewers to prove who they are or that they have actually utilized the business they are reviewing, Yelp review readers may be forming an opinion based on fraudulent reviews made up by a business's competitor, disgruntled employee, or an anonymous prankster.
Yelp has actually been pulled into court under a class action lawsuit and (criticized in the past for the byzantine methods) it uses to sort reviews on individual business pages. According to the site's FAQ, "Yelp's default sort order takes a number of factors into account and reflects our own attempt to present reviews in a meaningful order. For example, we'll favor reviews from your friends and the users you follow. The sort algorithm does not take into account whether the business is an advertiser or not."
By the same token, the "Yelp sort" has an insidious monetary element. Yelp salespeople have been known to call businesses with offers to push bad reviews far down the list, if they'll agree to pay to advertise.
In essence Yelp cheats both businesses and consumers through practices designed to benefit only Yelp. For a more reliable way to review a contractor before hiring them, it's best to turn to sources developed for contractors - like Angie's List, Home Advisor, and the like.