Well, as much as I wanted to like this lens I have decided to return it...
I did quite a few test shots comparing it to my EF-S 18-135 IS USM and EF 70-300mm IS II USM mounted on my EOS M50. With the exception of obvious increased softness at 400mm, it was pretty much indistinguishable from the Canon lenses at 18mm, 35mm, 50mm, 70mm, 135mm, 200mm and 300mm.
While it might be a viable option for someone who doesn't have other lenses to cover its range, the "all in one" aspect just didn't impress me as much as I thought it would. Additionally, the zoom ring is quite non-linear. It zooms smooth and easy and with expected resistance from 18mm to about 70mm where it hits a wall of increased resistance to push past to 100mm and beyond. Not sure if this is intentional or just on my copy but I found it very annoying.
Throughout the entire range focus was a noticeably slow and it seemed to have moments where it really hunted when it shouldn't have needed to. I even had to manually focus it on one test shot of a red rose with clear petals. Both the 18-135mm and 70-300mm Canon lenses focus faster and never had any hunt issues during my test shots.
While nice and compact considering it's range, it is thicker and only about 1/2" shorter than the 70-300mm Canon, mind you the 70-300mm IS II USM is an EF lens providing full frame coverage, whereas the Tamron is EF-S and the vignette is there throughout the complete zoom range. If you use it on a FF body it requires about 1.3x crop to get rid of. I think it is too big for a "walk around" lens to be on the camera
The absolute killer though was when being used on my EOS R in crop mode, it gave my camera a seizure - stuttery focus with fast short range back and forth without locking, erratic operation and after turning the camera off and back on, I found that all of my custom button assignments had been wiped out/reset.
Given the non-linear zoom pressure, focus speed, focus issues, and softness at 400mm I don't think it is a $650 lens. To me, it "feels" more like what I would expect from a $400-$450 lens. If I were to hold, examine, and play with it side by side with the Canon 70-300mm ($499) knowing nothing about them and going only on an initial impression, I would guess the Canon to be the more expensive lens, even considering the additional range of the Tamron.
So all things considered, it just was not the "one to do it all" solution I was hoping for and its shortcomings are not outweighed by not having to carry a second lens and occasionally swapping between them.
If you have an 18-135mm or even an 18-55mm kit lens and wanting to get something in the 300-400mm range, I would recommend the Canon 70-300mm IS II USM. While I didn't specifically test it, I would bet that taking a 300mm shot from the Canon to the same framing of the Tamron at 400mm, it would provide better IQ than the Tamron at 400mm.