It's pretty soft if shooting wide open from 17-50. Although it has higher MTF charts then the Nikon 17-55, it's kinda odd that pictures are clearly sharper on the Nikon. ( I pixel peep in PS and shoot raw).
However, stopping down to F4 = tack sharp for the Sigma so I might replace my kit lens for this. The 17mm length is very good.
Getting this lens for those low light instances might not be much of a use as F2.8 pics are going to be softer. I wouldn't say I am disappointed but I expected more for a EX lens.
I got around the rotating AF ring however, just finger clamp on the rubber barrel and it wouldn't be in the way.