For anyone wanting a telephoto for birds in flight or action sports, do not buy one without a focus-limiter. The Tamron will start focusing at the point where it last was focused, go back to minimum focusing distance, and then back out to the subject. By the time this happens, the subject has usually gone. If you require fast AF, get a lens with a focus-limiter.
This lens was not even a year old, when two parts of the focusing mechanism came apart. They were held together by glue, and the glue failed. What kind of quality construction would rely solely on glue to hold two moving parts together?
The optical quality was quite good considering the price. Images with the 1.4X TC were soft, and with the 2X TC they were unacceptable. AF speed was terrible with either TC.
If you want a quality telephoto lens, go for the Canon 100-400. 100mm less range, but superior to the Tamron in all other respects. If you need the zoom 400-500 range, I would recommend the Sigma 50-500mm over the Tamron.