I used the mark I and even that is pretty good with sharpness. of course there is barrel distortion at the wide end but not excessively so like the EF-S 17-85.
yes my 17-85 is a horror for group shots. some poor random person's face will just turn oval-shaped around the edges of the photo.
kinda off topic, but with a polariser + uv filter on 17-85, i have to go at least 25mm or so to get rid of the vignetting.