I think as Barry says, I have a poor copy, I picked it due to the reviews I read and not needing anything too fast. for the time being I'll stick with my 35mm F1.8 which thankfully is tack sharp!
Same as Mark (005269) for me. Read all the reviews and was dissapointed with the end results on a D7000, so sold it shortly after. Replaced with a Tamron 28-75 f2.8 which I found excellent on the D7000.