I tried a 3 month "sponsorship" with Yelp last year. For what we got, it wasn't worth it for our business (not a restaurant). While I'm not defending Yelp as a whole, I did learn that the reviews will disappear and reappear depending on the activity of the yelper. In our case, we had a horrible review that I think was fraudulent (I couldn't find any hint of a client that fit his story in our records). They wouldn't remove it when I asked because I couldn't prove that it was a lie. Thankfully, it disappeared after a few months. It reappeared for a short time then went away again. This person is a rare yelper so his reviews get pulled when he's inactive on the site. I wasn't harangued by the advertising rep, but I never did talk to the person in charge of my account once I signed up. I don't care because I didn't want to be bothered with her teaching me how to use Yelp, but still it was irksome. Our business has very few reviews so every one means a lot for my overall rating, but I can certainly see how a negative rating for a restaurant can scare some of your clientele away. Hopefully, as more people grow up with internet reviews, they'll learn that anyone's review is skewed by a single experience, their tastes and the reviewer's morals. Maybe each review will mean a little less.
Just my thoughts. Take them for what they're worth.