When the autofocus is true: Great sharpness and beautiful bokeh

After the great reviews and good reviews of this lens had irritated me and I had ordered it here at Amazon. And partly been promised too much. I have tested it with a Nikon D7000 and of its dimensions it has well together. However, it is considerably larger than my immerdrauf Lens 16-85 mm 1: 3.5-5.6, but not too front heavy. Where it does not hurt to have a battery grip, then it is better on the balance here. In weight should one make no illusion: It corresponds felt a brick, but that's normal if you want to have a high quality finish with high light intensity.
The zoom and focus ring are easily accessible and luckily the focus ring is positioned forward. Thus, you can zoom even if the lens hood is placed upside down. This example is in the 70-200er of Sigma not the case and thus can be very annoying.

--- Positive Characteristics ---
-A Beautiful bokeh over the entire Brennweitenbreich (see photos)
-Very Sharp (I would say even sharper than my Nikkor 50 1: 1.8) and 1.8 -> really impressive
-Top Processing (similar to the Nikkor 70-200 and significantly better than the Sigma 70-200 1: 2.8 OS) and not comparable with Sigma lenses from earlier series.
-Leiser And faster autofocus
-mechanical Top -> as manual focusing is really fun

--- Negative Features ---
-The Auto focus is not always accurate:
Especially in darker rooms he had to ask properly focused especially at 18mm problems and often a tendency to back focus. Even with the internal AF correction of Nikon himself was not satisfactory resolve. With 35 mm, however, it has almost always fit. The behavior I could reproduce on the tripod. Especially with the lower focal areas there were problems. I have the scenario with my Nikon lenses 70-200mm VR 2; Tested 50 mm 1.8 and the 16-85 mm and in all there were in the same proportions no problems.
-The Lens cap has deteriorated, in my view towards 70-200er from Sigma. Because the placement is quite hakelig when plugged lens hood. Is not really dramatic, but something has disturbed.
-Was Before buying known: A seal on the bayonet would have been justified in this quality (as in the higher-quality Nikkor lenses). It would also be great if you could intervene manually at active AF.

--- Conclusion ---
A great lens when the autofocus would be more accurate. Some would say now that the USB Dock could buy yet. But in my view that is of interest to the fine-tuning and should not be a prerequisite for a functioning lens. And so one objective should therefore focus well right from the start. But apparently the AF subject is still a problem in Sigma. A similar behavior has also been described in dpreview.com. Unfortunately I had a bad experience with the Back-Focus even at a Sigma 70-200 OS 2.8.
Possibly I let myself be carried away even to test a second copy. If so, I will describe the experience here.

--- Update 04/23/2014 ---
So now I had to Easter yet again decided to try the Sigma and had ordered a new copy. This time I have the USB Dock ordered to any inconsistencies to be able to compensate for the same. And unfortunately, the disillusionment came quickly. Also in this instance the same problems, that is, in many cases, I was struggling with a back focus. Accordingly, I had the lens connected to the USB dock to adjust the focus point for the appropriate focal lengths. This can be set up quite easily and is quickly transferred. In most cases, the back focus weakness could eradicate. But unfortunately there were still problems with 18 mm in distant subjects. Because here there arises a front focus. And so a case can not be corrected cleanly over the software calibration. Because depending on how I had set it, there was a correct Schräfe at 2m distance and a front focus at infinity. Or I had a back focus at infinity 2m distance and a proper focus on.
After two hours of work, unfortunately I had to realize unnerved that the lens not be calibrated in such a way so that it is properly focused with all focal lengths and different motif distances. Thus, the nothing with my D7000 and Sigma. It is such a pity that does not work properly, because if it is time you get great sharp pictures.
According to this new experience, I have down graded the rating of 3 to 2 stars.