Yelp is strong in our area - it's interesting though... their reviews boost my Google organic listings, so I get Yelp referrals through Google searches by NON-yelp members who click the review link... but the direct referrals through the actual Yelp site of Yelp members have an average invoice half my normal average invoice. They are interesting metrics.
I talked with their back stage department today about their ranking system and advertising. One of their filters which I know many are subject to is that new member reviews are automatically filtered out. They wait until a new member posts a few reviews and sticks around awhile. This is because of what we see in our industry especially, competitors and boneheads posting fake reviews to malign a competitor... so this setting of filtering out the new member reviews catches the good and bad ones.
But... once your customer is around a bit, and active, the filtered reviews do become active.
Yelp is adding a million new subscribers a month, and at least in California, they are kicking Angie's butt. But Yelp started in SF, so it's growing here first.
It's still a toss up though... these referrals - at least the direct ones, tend to be a younger crowd, and much more price focused than my other referral sources.
Lisa