hiya, just joined the forums for this thread. Bought my 50 art 2 weeks ago and have been trying it out in and out of studio and on my commercial shoots. First 5 shots I took at 1.4 outside the sigma store in shanghai, 2 were completely out of focus, it wasn't even an issue of focus shift, like NOTHING was in focus. The other 3 were amazing sharp at 1.4. In studio, half body and closeups were amazing at f8. Full body shooting from 4 meters away with a 200w modeling lamp, the lens would focus 1 feet in front of the model 50% of the time... adjusted +6 in a 5dmk2 and single point focus, the shift was much better but still some 'soft' images. Outdoors in bright sunlight almost all images were dead on. I thought I had a bad copy and was going to swap it, but by the looks of it I'm not going to bother.
I still think i'll keep it as when the focus is right, this lens in incredible.. but I won't be using it for my commercial work much as the inconsistency is way too much.