Based on some praises here in CR, I've apparently had bad luck with this lens, purchased new from a big online camera store. Could not get it to focus properly on an 80D, the camera I had hoped to leave it on, though other comparable quality lenses (ef-s 35mm f/2.8 IS macro, old ef 85mm f/1.8 USM, ef-s 24mm f/2.8 STM) do great on the same body. Wouldn't focus well in Live-View at all, though with a mere -5 for AFMA value I got pretty close to centered for phase detect.
But then it just wouldn't get sharp, again compared to other similar quality lenses, and the AF is not reliable. Pretty much the same dodgy AF as I experienced with the Sigma 50mm f/1.4 Art a few years back on a 5D III.
The CA was MASSIVE, purple and green so bad it tinted numbers in AFMA tests, and looked like stained-glass windows in high-contrast backgrounds. Made the 85mm 1.8, in this regard, look amazingly free of CA in comparison.
The IS has noticeable drift while on a tripod. In other words, with IS turned on, the center point of the lens will slowly move either southeast or southwest quite a bit, and keep drooping while Live View is active. This may not be a problem, as turning off IS on a tripod is best practice, generally, but I've never seen this happen with other IS lenses.
Build quality does not "feel" impressive, but like a cheap kit lens. The silver ring around the base is tacky and toy-like.
Finally, the AF was unacceptably slow, much slower than described in reviews. Even for small changes in distance.
Did I get a bad copy? Maybe. My first Tamron ever. Nowhere near the gem reviewed. But there must be a reason the price has drifted down to a virtually permanent $400. Perhaps quality control slipped? It would be a rare day indeed to get a lens much better than the retail price suggests it should be. I didn't.