I have a 17-40mm and am ok with the focal length, however I cringe at the corner distortion. It is like when Fox TV tries to widen a SD feed to a 16:9.
Yes corner sharpness needs improvement... that goes without saying, but in group shots, it is the distortion that bothers me the most.
I would like a 16-50mm f2.8 so that I am carrying fewer lenses when I go trekking.