Here are my observations after using both Canon v1 and Tamron 24-70 VC.
Pros
VC is just amazing. Tested 1/5 @ 70mm with close to MFD and it works like a charm. Just amazing.
Tamron USD AF is just so super quiet and so is VC compared to Canon. You just don't know it's working.
Color and contrast are at par with 24-70. Maybe 24-70 is tad better, but that all can be manipulated in PP.
Build quality is very good. Not like the brick, but very good.
6 year warranty. Can't beat that.
$11Xx something price is very good.
Corners are similar to 24-70 v1 wide open. I think it's a wash and a pro more than a con. I think @5.6 it's better than Canon.
CA is a wash wide open. Again a pro for the price.
Cons
Focus is definitely slower than 24-70 v1. Might be very small difference, but there is and this will put off many people.
Vignetting is heavy @ f/2.8 compared to Canon. This was kinda expected, but more than I'd wished for. Can be easily corrected in PP.
Image jumps all over viewfinder, I guess this is a known issue.
If you want sharp corners wide open, please look at canon v2. Some people give a damn. I don't personally.
82mm filters are expensive.
Resale value will drop to $900ish at some point unlike 24-70 v1 which hold their value even now.
Screwed my metering couple of times. Might be anomaly or user error. But was weird why it did that.
Let me know any questions you might have. Good luck.