Rich Skrenta of Topix.net discusses both the percentage of readers using RSS clients and the popularity of specific clients.
Some data from Rich on clicks per story for users of specific clients:
Newsgator, 29%
Feedster, 24%
MyYahoo, 20%
Intravnews., 4%
Netnewswire. 4%
Pluck, 3%
Sharpreader, 3%
Feedreader, 2%
Feeddemon, 2%
Firefox, 2%$

Bloglines, Rojo and Amphetadesk--three I use regularly--are all MIA from this list--as is Technorati, whi, Rich says, doesn't seem to be crawling Topix feeds. (why the H not?)