I'd love to have this lens for my Canon (in fact I had one, but returned it because of one soft corner), but I'm having doubts...
You tried once?
I can't believe that Tokina (and Sigma, and Tamron) really carelessly manufacture and ship them with sharpness issues. I rather believe that their internal construction isn't well designed, so the lenses get loose sooner or later.
I think I can say without hesitation that Canon also could fall into this category, along with everyone else depending on the model
If that's the case, also a sharp sample wouldn't be the ultimate solution.
Does anybody have reliable information to either support or object my thesis?
Reliable info? I'd love to see some sort of database! Personal experience is all I got here - as well as having had a photo lab/studio for a while, so we did have an awful lot of equipment.
I think that as far as 3rd party goes, you always run a risk of not getting a good copy right away. Probably the largest reason for that is that they make lenses not for a single manufacturer, but a bunch of them. So, besides having to deal with the mechanics of the optics, they also have to worry about the electronics, and are subject to the whims of the manufacturer. Canon only has to worry about Canon.
Back in the old days, we used to have to ship lenses back to Nikkor frequently. It was just part of doing business, and since I wasn't personally involved with the equipment it never really bothered me. I just wrote the checks. Heck, everything we had needed constant tune ups or tweaking or replacement, from printers to the processor and mounting machines. I was on a first name basis with all of the tech guys.
But, back to being a lowly consumer - I really think with today's amazingly high manufacturing tolerances, once you get a good copy it's quite likely to stay that way for a long, long time. If, that is, it's on the higher end of the manufacturer's product line. The plastic built EF-S lenses for instance, I wouldn't really expect one of those to do a daily grind and survive 5 years. An L lens? It better. I've had my "consumer grade" Sigma 10-20 for 5 years this February (I just noticed I get most of my equipment in February!). It's been used almost daily, and has tens of thousands of clicks on it. It's just now, and I mean just now looking like it might be soft in one corner - and this is the 3rd body it's been on.
All of my other lenses are fine, all the way back to Nikkors I bought in the 70's. They don't have nearly the mileage of that little Sigma. It's proven itself so much that I've gotten 2 other Sigma lenses. Then again, my EF-S 17-85 was supposed to be a dog that would break down - and it's still going strong with easily as many clicks.
Then again, I got a Tamron 70-300 VC, and burned through 4 copies. From DSAA (dead shortly after arrival) to corner softness to focus issues at certain focal lengths. That's really the worst experience I've had in years and years - and I'd still give them another shot! I liked the lens a lot - and would love it if it did everything it's supposed to do. It wouldn't stop me for instance from seriously looking at their new 90 macro, or the 24-70 for my (soon to be) new FF. February is coming up you know!
Now, that Tokina 16-28 is also one I've eyed for FF, as I need such a WA beast. I would not hesitate for a moment to buy it if I liked the handling, and I got a good copy. Especially for the savings over the EF 16-35L f2.8! Bottom line though - for the savings I would not be too upset if it needed to go in after a few years for calibration. In the same vein, don't be too upset if you have to send your L in after a few years either!