If you’ve never visited it, you should spend some time at Yelp. It is a Web site devoted to local business reviews, where more than 20 million unique visitors monthly go to read and write about their personal experiences with restaurants, stores, dentists, car mechanics, etc.

The site was founded in 2004 and contains more than 5 million local reviews. Yelp was not the first site to provide the opportunity for consumers to review businesses; most of the Web Yellow Pages and business directory sites have had that feature dating back to the late 1990s. What Yelp did differently was to fully embrace social media, encouraging its reviewers to establish personalities and become featured contributors.

Reviewers on Yelp generally complete a profile of themselves that includes photos and links to friends. Yelper profiles contain links to all of their reviews, provide their ratings distributions, reviews of their reviews, and much more. It’s a mashup of Facebook and a visitor’s guide.

However, creating a new category – as Yelp has – does not happen without growing pains. Just this week, a California court ruled that a dentist could continue to pursue a lawsuit against a husband and wife who allegedly defamed her by yelping about the poor treatment she gave their son several years ago.

And a growing chorus of small businesses have complained about Yelp’s advertising sales tactics. Advertising on Yelp comes with the right to elevate positive reviews of your business, thereby deemphasizing (but not eliminating) bad reviews. But there have been a number of news stories written in recent months about merchants who are suspicious about good reviews disappearing and negative ones appearing when they didn’t sign up to advertise with Yelp.

Yelp’s coverage is understandably deeper in major metropolitan area such as San Francisco, New York and Chicago, than it is in less populated and rural settings. But I found many reviews and listings for Morristown, NJ, where I live today, as well as my hometown, in the Hudson Valley of New York.

At the very least, if you run a local business you should visit Yelp and see what the Yelpers are saying about you. As one very good marketer once told me, the only bad review of your business is the one you ignore.