Comparing location apps.

Has anyone here done any analysis comparisons between Gowalla, Brightkite, Foursquare, other location-based apps? I love Gowalla, but what is the attraction of the others that seems to get people to use them? Someone mentioned FourSquare being "easier" and I heard on another board that BrightKite had a "simplicity" that he liked. But how is Gowalla "complicated"?