Hey Mark,
I finished a usability report on the Foursquare app last week (attached). The key results are on pages 17 and 22 onwards. In particular I looked at:
- the check-in process
- location of the Friend's map
- searching for tips
- requirement for deleting check-ins
- accessing the leader board
I evaluated 7 people (that's all I could afford!); 4 heavy foursquare for android users, one foursquare for iPhone user, and two who use neither, so as to gauge "learnability".
I measured the number of taps and time to completion for a variety of tasks. The report culminates in a Criticality Rating which prioritises the features which were felt to be "Very Useful" and used "Very Often", i.e. most critical. There's also a series of quotes on an extended set of features.
More than happy to discuss, hope it's useful!
Cheers,
Nick