Slander vs. Opinion

What do you think? A user writes an opinion about a doctor or a plumber they have used in the past. You would think that the review would be an honest depiction of what actually happened, or is it.

This is a case where a user's review on Yelp has come back around to bite them in the ass:

http://tinyurl.com/cg7cw9

Take a look and let me know if you agree that user's (and Yelp in this case) should be held accountable for what seems to be obvious misconceptions on both sides...