I recently started trying out this new Four Square location check-in App on my iPhone to see what the craze is all about. After one week I have checked in at 12 places ranging from my school (Babson College) to Fenway Park (I was actually outside, those tickets are still a little rich for my grad-student blood) to the barber shop. At this point I don't have any "Mayor-ships" (I think you become the mayor of a place if you check-in more than anyone else). The UI is pretty good but occasionally it has some trouble finding my current location in its database.
From a marketer's standpoint I think it has some interesting possibilities, namely it could enable marketers to advertise to people in the immediate area which could be great for a small businesses like restaurants. When you "Check-in" you can see "Specials Nearby" which usually pops up some Starbucks ad but once it catches on I hope to see more local-based advertisers.
One other feature I like is the "Nearby Tips" which opens recommendations for nearby places similar to a Yelp-type app.
Here's another good blog post on location marketing and where it's headed: http://tinyurl.com/2elax6v