I payed for Yelp marketing. They boosted my business listing to the top of Yelp ads in a rotating fashion. They said I'd get X number of new clients each month based on some formula they used. Not only did I not get a single client in 6 months, but I didn't get a single phone call for new business. My other marketing avenues yielded results. Yelp did not.
Also, when you sign a 3 month contract it automatically rolls over into a month to month, which they don't tell you up front. If you choose a 3 month option make sure you actively cancel it one month prior to expiration. My suggestion is to avoid Yelp altogther.