Summary: Required factory focus recalibration. Still slightly front focusing, most noticeable wide open when there's little DOF. Re calibration process was painless. I paid to ship lens and camera body to sigma in Long Island NY, and they took it from there. Got it back in a few working days. I used downloadable focus charts to confirm issues, which improved. I compared it to my pro photographers canon 50 1.2 L on a tripod and we found IQ to be subjectively better with the sigma.
Pros: Image Quality Bokeh Fast, Nice Bokeh, Strong Construction
Summary: I'm using this lens on a Canon 6D. The sharpness is excellent and the smooth depth of field very attractive. Focusing is quick and accurate but not as quiet as the Canon 24-105. The lens is well built and feels nicely balanced. Image quality is superb.
Best for optics and price as long as you can get a good autofocusing copy
3 December 2012
Summary: I was weary to buy this lens due to many review of the autofocusing not behaving. I bought 2 copies to hope that one would work with my camera. I noticed within the two copies that one was softer than the other. The autofocus is also not super at low light situations. *** My previous Canon 50mm f/1.8 II had better autofocusing than this Sigma.