I've had the lens for some time, and sent it back to Tokina for a binding problem when close focusing. Supposedly under warranty, but they didn't honor it. Now, intermittently, it will not focus. I move the AF-MF switch. or turn the camera off, then on again, or remove the lens and replace it on the camera body, and most of the time, it will focus. I've tried cleaning the exterior camera and lens contacts, but nothing seems to help. Now; it will cost me a minimum of $120 to have Tokina do whatever they may do to it, and with the money I've already spent, I could almost buy a new lens. With all the positive reviews this lens has, even from the Angry Photographer, I believe I may have bought a lemon. I like the 150mm field of view on my APS-C camera, somewhat equivalent to the Sigma "Bugma", but am a bit hesitant to go into that dark alley again. Money is limited, so more expen$ive lenses aren't in the cards.
Suggestions? Comments?
Thanks!
WHOA! Let me back this thing up, here! I tried looking up the official user manual on line, but it was too small to read, even with a magnifying glass! Soooo...I got the lens our of my case, and put it back on my camera. Before I did, I gave it a good looking over...Aperture set at maximum, but What's THAT? 
A switch, small, but of exquisite design...hiding, yes, hiding a little green dot! Hmmmmm, sez I. What do it do? It are the Aperture ring switch, without which the entire works get farbled. So, i set it so the little green dot was showing, locking the aperture ring securely, and Voila! It works just fine! I can't blame it on some malign spirit. I just hate it when it turns out to be user error! 