I had purchased this lens from my local camera shop and was disappointed when I discovered that it would only focus on my 5D Mark III body. It would stall and quit trying to achieve focus on the 7D if I had focused on a near object, moved to focus on a far object and moved back to a nearer object. I took it back and tried 2 more copies of the lens but all had the same issue. That night after trying to solve it by carefully looking at the 7D setup, I realized that Custom Function III Option 4 was set to stop the lens drive if focus couldn't be achieved. I reset this to "1" and then went back to my camera dealer to try the Tamron again. Problem solved!
I've seen quite a few posts with similar problems on a couple different Canon bodies. I don't know if what I did would fix those issues, but it certainly took care of me problem. I am still just now trying the lens (I just got it today) but so far I am very happy with the focus and speed of focus.