I use this lens 7 days a week, so you might not be surprised to hear that I am now on lens No 3.
This is because the iris motor will eventually give up.
The first one lasted for 5 months, and the second 9 months. So why do I still use the lens, because the results are clear sharp images and works very well for the price, yes you do loose a bit at the top end, but unless you are in a position to buy a very expensive L series lens, then this lens will suit both pocket and need.
I now write this 11 months on from the first review of the Tamron 70- 300mm.
Lens No 3 has just died, same old fault, Iris motor failed when the lens is at max zoom.
As much as I still do like the quality of image produced by the Tamron lens (crisp sharp image) I can no longer justify buying another replacement, three failures is just too much. So it's time to bite the bullet and buy a more expensive Canon (with a extended warranty)
If you will be a light user of the Tamron 70 - 300mm, then the problem should not show for several years, but if you use regularly (not professionally but a keen amateur) then consider the above information before you part with your hard earned cash.