I mast say that after over twenty years been a Canon shooter I have shot with just about every lens ever made by Canon, and I have also shot with many other lenses from Zeiss, as I'm also a Filmmaker, I have used them both on Digital Cinema cameras like the RED ONE, as well as use them on my Canon cameras with adaptors.
I try to avoid zooms all together, even so I must say that lately the zooms are getting very good, but still not as good as primes.
Quality issues have been experienced in various lenses, and both Canon and Zeiss, have had their share of returns, more so Canon, I guess as they are getting bigger and bigger and their sales going in much higher number their mass production of lenses, and service facilities have to deal with, and I think that it is an absolute must to purchase a lens only if you can return for replacement of =r a refund, incase of purchasing it used form andy of the forums.
I agree that is difficult and time consuming to test each lens, but unfortunately has become a necessity, at list for me as a Pro shooter, you don't want to take the gear to a job to find out the all of your pictures are not good, do to extreme corner softness, or a lens being de-centered, or what else, which have now caused issues and problems in your work, some of which could be fixed, but giving added work to do, or even not fixable literally screwing up your day.. or two.. or many more depending on how long will take for a replacement, and now that you have experienced first hand the issue in quality control you will think two times over before taking a lens for granted, not all lenses are created equal, unfortunately is the true, so test them out.
