I got this lens after reading raving reviews on this lens on the web. First copy I got had a problem with accurate focus. Tested the lens on a tripod, mirror lock-up, timer, and center point focus. I got hit-and-miss about 50% at the wide and long ends.
I exchanged the lens and immediately tested the second copy. At 17mm, focus was spot on but at 50mm focus was again 50-50 hit-and-miss.
Again exchaned for a third copy and redid the test. Still at 50mm, the focus was not accurate but was a bit better than the second copy.
The moral of the story: if you got a good copy then this is a good lens.
Another thing is the noisy sound when it is autofocusing. I read that people were bothered with this sound and I thought to myself that it could not be that bad. But turned out it was pretty loud and not too pleasant, at least for me.