Similarly, the problem they reported at lensrentals was announced via a special blog entry, but it took a rather obscure response to a specific comment to report that subsequent copies of the lens are just fine and have no new problems.
3rd party manufacturers like Tamron are at the disadvantage because they have to fight a bad reputation, esp. if releasing a quality lens like the 24-70/2.8. If a Tamron breaks, surely the cheap quality is to be blamed, if a Canon breaks it's really, really bad luck and could happen to anyone.
Sure, but straight out of the box the lens should function as claimed. Tamron hasn't "earned" the bad reputation for nothing.
I won't say that I've not had problems with Canon, but the problems with the third party lenses have been (a) more frequent, and (b) are compounded by lack of customer support because (usually) Canon and the third party manufacturer blame each other and refuse to take responsibility for the problem.
I'm am comfortable giving my lenses and bodies to Canon for calibration ... I'm not too sure about Tamron.