Popularity isn't always a good thing as McDonalds is wildly popular just because it's cheap, fast and consistent, but I wouldn't give it a 10 rating except on the service at our local drive-thru which easily earns a 9.5.
Four Square should also be more interesting because the average person can't game the system like they can Yelp. However, all you need to know is how to spoof GPS in developer mode so enable "mock locations" and get a big list of proxy IPs and let the gaming commence.
FWIW, I frequent a lot of places I absolutely love and have never "checked-in" via 4sq, Google+ or anything else to date, not even to experiment with it. Not that I'm being overly paranoid, as I'm not a big internet privacy freak, but telling people exactly where I'm at in real-time is a bit over the top IMO unless you need help in an emergency.