I had the opportunity to test WinMoSquare (still not launched), the Foursquare Client for Windows Mobile. I was pretty happy when I got the .cab because this application seems to be the only windowsphone-client for Foursquare. In the first run I could not check in or search for my friends due to non-US-Geo-location-issues of my Sony Ericsson Xperia X1. I sent the logfiles to the support-team of http://www.touchality.com and got a new version same day (great job!). Now the app is up and running. I enjoy the user interface (see screenshots), the list of venues (any kind of locations, mostly restaurants) next to me (GPS-signal of my mobile is used). I haven`t played with the iPhone, Blackberry or Android-Client, so I cannot really compare.
I would like to have this feature: show my friends which are next to me in a different color or just as first in the result list
In short: It seems that I won`t go back to http://foursquare.com/mobile/ and use this client instead.