As a web service, Yelp is pretty good. Want to find a good thai restaurant in the mission district of San Francisco? Yelp recommends Racha Thai, with a rating of four stars and 32 reviews. How about the best burger in Boston? Yelpers would agree that Canestaro is the ticket, with 4 and a half stars and 33 reviews.
In addition to restaurants, Yelp has a pretty good database of shops, nightlife and beauty spas - for most US cities. So you might think the iPhone app would be killer. It's not - at least yet.
Amazingly, the "Yelp app" does not allow you to filter by type of cuisine, only by the distance to you or the neighborhood. So the app is pretty much useless in helping you book a sushi restaurant in LA. The restaurant profiles don't link to the restaurant's websites, or to Open Table, making it that much harder to book a reservation. The map doesn't have a "directions" mode, which other apps do. And other aspects of the website, such as "related searches" are not present.
I'm sure this site will improve, but for now, Appcraver prefers UrbanSpoon in the food category.