I had serious AF issues straight from the start. When I showed it to the shop in which I bought it, they recommended I should send it to Sigma. I did so, and they returned it saying it was "within specs" (no data shown to prove it) as they had tested it on one (1!) body identical to mine. The front focusing was so bad it was impossible to compensate for it with microadjustments. Bought another (better/newer) body a few years later and I have identical issues with that lens on that body. Still not possible to compensate with microadjust.
Sigma mentioned that they might be able to fix it if I did send both body and lens to them, but I certainly don't trust them enough to have my cameras. Other shops hinted that Sigma might try to adjust my Canon house to work with that lens, and it would most likely screw up all other lenses (all of them Canon) I have.
All in all I ended up with an expensive door stop that takes super sharp, crisp, photos on things I don't focus on, and I had to pay for sending it to Sigma, who showed no signs of doing anything to fix my lens or to compensate me.
I wouldn't buy another Sigma lens unless I can test it on my camera for a week and it delivers a reliable, and expected, result during that time. Much much easier to just get a proper lens from the start, that is manufactured by someone that cares about photography, their customers and their brand.
I took a few shots with my Sigma 50/1.4 yesterday. Here's one properly processed with focus as I wanted/intended it to be, and another one that is just a screenshot showing a crop with an overlay of the selected focus point.
That last one is so so bad that even your guide dog would have to be blind to pass a focus check...