Got my 50mm 1.4 art one week ago (Norway, FotoVideo.no)
After one week of use with 5d mk III my intuitive impression was that focus accuracy was quite OK at infinity, but not perfect. However, at closer distance it was very much off.
Using the USB-dock I experimented with test shots for a couple of hours, ending up with the following adjustments:
0.4 m : +15
0.7 m : +14
1.5 m : +7
I based the adjustments mainly on comparing live view focusing with normal AF, trying to make focus hit the same way with mirror up and mirror down. Live view focus with mirror up was of course extremely consistent, but slow. Normal AF with mirror down varied a bit in some test shots, but not noticeable more than what I was seeing when calibrating focus on my Canon 70-200 2.8L IS mk II.
As you probably realise, the adjustments I ended up with is not something that could have been corrected only by micro adjustment in the 5D mk III.
Portraits shot near 0.7 m and 1.5 m seems to hit with focus around 75% of the time for me at aperture f/1.4, after adjusting focus with Sigma USB-dock. Before adjusting it was not focusing well at these distances.
I have only been shooting in good light, although with heavily back lit subjects (shooting more or less into the sun).
the 25% misses are probably my mistake, dealing with moving subjects or me moving. The misses on the 25% were only slight misses, with somewhat blurred eyes, and quite even spread where sharp focus had hit either just behind or just in front of the eyes.
DoF at f/1.4 is so narrow that subject motion or camera motion easily makes the shot OoF.
I only have one other lens, the Canon 70-200 2.8L IS mk II. For me there is no practical difference in focus speed and accuracy between that high end tele zoom and the Sigma 50 art. Shooting at f/2.8 at 200 mm gives me about the same focus hit ratio (75%) with the Canon and the Sigma 50 art at f/1.4, when shooting at high enough shutter speeds to eliminate camera shake and subject motion blur. Both lenses has been focus micro adjusted. Probably, I am guilty of some slight recomposition after focus confirmation, risking an OoF shot while trying to improve image composition.
As I said above, I think the 25% misses that I experience is mainly due to subject motion or camera motion in the time from focus confirmation to shot taken. It is really difficult to judge when not using a tripod, and when the subjects are people that move around a bit. To me, the AF and sharpness in Sigma 50mm 1.4 Art lens really gives confidence so far, just like my Canon 70-200 2.8L IS mk II.
It is fantastically sharp at f/1.4, and gets even slightly sharper when stopped down a bit :-). I will probably use it at 1.4 most of the time, as it is more than sharp enough for my use, and I like the blurred backgrounds at this aperture.
Just keep in mind, if I had to use this lens without adjusting focus in Sigma USB-dock, I would have been extremely unhappy with the performance, and probably would have returned it within the first week of use. In my opinion, it is no point buying this lens without having access to the USB-dock, as it is a very high risk of focus inconsistency before correcting it in the USB dock.