I don't have the lens. With landscapes as a main interest, I thought that f4 was fast enough and I did not need the motor - thus the 16-45 was just fine and is smaller/lighter. Why I am posting, is that the lens was co-designed with Tokina, and I have always wondered if Tokina had the same/similar problems. This time I actually went out and looked around a bit.
Tokina discontinued their version about 2 years ago. I saw a number of references that the 16-50 to folks was somewhat of a disappointment in terms of softness versus their experience with Tokina's 12-24 (another co-design with Pentax). The general feeling of the Canon / Nikon folks was that the Tamron 17-50/2.8 was a much better lens all around.
There seems to be two camps - folks who
have it, like it, and have had no problems with it. The others are folks who
had it, found weak performance by f/2.8 and at 50 mm focal length, enormous chromatic aberration at 16 mm, high vignetting, noticeable coma, sold it and went to Tamron.
It appears that of the set of lenses co-designed with Tokina [10-17, 12-24, 16-50, 50-135] (plus Tokina took the 12-24 and leveraged the design into the very successful 11-16), the 16-50 was the most problematic of the group.
Also, and I think that this was to be expected, I found very few problem posts with Tokina's lens motors. It appears that the motor problem is somewhat isolated to Pentax. I did find some references to Nikon lens motor faults. It was also interesting to find that the initial version of the 11-16 was screw-drive, with the second version having the focus motor added.