i have the 21, used to have the 15 but sold it.
personally, i found the 15 too difficult to use, hard to think for it's field of view. i got some great shots with it that wouldn't have been possible with any other lens, but it was hard hard hard to get good results. it ended up sitting in the bag, or just not getting taken places.
the 21 is a magic lens. very sharp at f5.6 or above, minimal vignetting (sp?) unless you push it, minimal distortion. great field of view for landscapes and architecture. and the vf is very very good too.
all of which just echos a lot of what others have said
personally, i think the 21 is one of the best cv lenses.