When the Sigma 35 Art is well focused, the IQ is very good. But the AF is not consistent. When I got it last summer and the days were long and light was good and I normally used f5.6 or thereabout, it was excellent. In November days are shorter, light dimmer and f-stops wider and I got more and more out of focus images. I did a new AFMA (Focal) and focus had drifted 7 points. Whether that was a one-off or something that I can expect to happen again is yet to be seen.
My Canon 24-70 f2.8L II has been outstanding from day 1. I have not had any of the problems other report. Like some other posters here, I am also a bit reluctant to buy non-Canon lenses, due to reverse engineered AF solutions. Sigma does seem to have something going though.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1