When FS was "unavailable" in the mktplace for a period of time, 4th&M really took advantage. Not many have switched back for some reason. 4th&M for me. Just following the herd, I guess.
I use 4th & Mayor. When I first got my WP last month and started using FourSquare I used only the official app, but read an article somewhere on 4th and Mayor and haven't looked back. Love the app.
I like the look of FS more, but the functionality of 4th makes it my go-to app. I have 4th's "Check In Now" pinned to my home screen. I don't really use it for anything else other than that and checking specials, which is easier to get to on 4th, since it is my second app on my all apps page (smart move, Wilcox, haha).