I never check into restaurants -- I don't see the point. But I regularly check yelp to see reviews of new restaurants that spring up. Then I go check out the new restaurants with good reviews. (I live in Chicago.)
In Chicago, and cities of that size, this makes a lot of sense. It's a huge city. You can live there for decades and still not have a handle on everything. There's too much for word of mouth to render Yelp unnecessary. I'd use it, too, if I weren't in a city 1/10th the size of Chicago.