I think I heard about this site a while ago and probably thought it would be a great idea - but only if they can get enough people to actually take the time to submit reviews and reach a critical mass. Recently, reviews on Yelp have started turning up in Google search results and I've found that there are now enough reviews for them to be very helpful when choosing where to eat or shop. So, I signed up myself and found it quite easy to get addicted to submitting reviews for all the neighborhood places I frequent. The mapping feature is quite handy - the review page gives a small Google map of the area, highlighting the place being reviewed along with 4 others in the area. This makes it very easy to discover new places that you haven't been or remind you to rate those other nearby places that you have been. They're also doing a decent job trying to build some community around the site.
If nothing else, its a convenient place to store all my own reviews so I remember what I liked or didn't like. If others get benefit from this, then all the better!