I really want a Tamron 24-70 f/2.8, but this kind of report does put me off. One round of servicing is fair enough I suppose, but four?!
I had one before which was pretty good. It was stolen in January. So I bought a Canon f/2.8 24-70mm Mark I, but wasn't satisfied with the sharpness at 2.8. It also has a lot of CAs in the blurry part of pictures with backlight, so I've got a Tamron too. The first two times it was at service for adjustment, the first one failed, so I got another chance on warranty.
The third time it was there, because the left side of each picture was quite extreme out of focus. In most pictures you have sharpness in the center and you don't care about that stuff around, so it takes a while until you recognize that.
Now I found out, that now the left side is out of focus, but more slightly and probably not visible at other than f/2.8. But I came to that, by checking how good the outer AF points do work and as it is, I get the focus too much in front when using the outermost left point and a little bit, when using the outermost right one. Both are working perfect with the Canon. So I'm considering to sell both after the Tamron is adjusted again and buy me a Canon Mark II instead. On the downside the latter has no IS, which was another reason to buy the Tamron. The Canon 24-70
f/2.8 IS is rumored to come out in 18 - 24 month and will be for sure too expensive anyway. And the 24-70 f/4 IS is not much an option, although I barely use f/2.8 on the 24-70mm, but it's more psychological.