Initial testing showed either a front focus in close range, or a back focus in mid-range, depending on which settings I chose in the USB dock. And somewhere between 12 and 14 meters, the lens would never focus perfectly. I also found the AF to be not 100% reliable in other ranges, but in 12-14m it never focussed perfectly.
I sent the lens to Sigma service. They updated the software, and told me to do the fine-tuning using their USB-dock. I did that but to no avail. So I returned the lens to Amazon (I'm still grateful to them, I explained everything and they took it back without problem) and got another specimen, which showed the same symptoms, but decidely less pronounced than the first one.
I decided to accept the front focus in close range as I never really use the lens there (it's in the range of around 1.20 or 1.50 meters), and adjusted the lens accordingly in the USB dock. Since the probs were a lot less pronounced in this specimen and I liked the images a lot when the focus was right, I decided to keep it, stopped 'testing', and went on to normal use.
I never use the lens in the range where the AF would produce a front focus, so the focussing works well for me, except sometimes when targets are moving towards me fast I don't get perfect results (but still a for me sufficiently good rate). And that slight fuzziness in the 12-14m range hasn't affected any of my images at all.
I have not gone back to testing to see if I can still reproduce that unsharpness in that range. I don't think I made a mistake during the testing, I think the way I use the lens, that slight unsharpness in that range simply does not matter as it was too insignificant to be easily detected in real life images. Or maybe when adjusting the lens in the USB dock the final time, it also went away.
Edit - Maybe I should add that a few days ago I took a close-up picture of a small twig protruding from the surface of a lake, and that twig was tack-sharp. So I really don't know whether in my testing I did something consistently wrong, I really don't know what to think. But the lens works like a charm for me now.