I try to take a grain a of salt and look for a common theme, compliment or problem in reviews. Everyone knows there are morons in the world who will think shit is great and great is shit.
My personal opinion and that as a business owner is that it's perfectly fine for people to comment about your business honestly. I've gotten quite a bit of work BECAUSE people have posted about my work or product online or in topic specific forums.
The biggest thing I see is that there should be a space for the business to respond on the major sites. If someone buries you, you should have a space to contradict or explain the problem. Sometimes the best solutions is to ignore it and just work hard to get enough positive clients to drown out the hater.
"Silence is Golden. Duct Tape is silver"