We all appreciate honest and informative reviews, both positive and negative but watch out, you could be sued if you're not careful. There is an article going around that describes a Virginia women being sued for $750,000 for posting a review on Yelp because the contractor she criticised considered it defamation and claimed that it damaged his business. It will be interesting to see how the case turns out.
If your criticism is reasonable, honest and accurate it's unlikely you'll have trouble but if your intent is to damage a business you could find yourself in need of a lawyer.
Here's the article:
http://www.washingtonpost.com/local/...f15_story.html