For me, the big question is how well autofocus will work, particularly since most Canon's have an F5.6 AF limit.....
All of the 3rd party f/6.3 'spoof' the camera into thinking they're at f/5.6, so AF works fine. Well, I guess I should say Af works as well as possible, given that most 3rd party lenses aren't known for having good AF performance.
For me, the optics are the big question. I know 'world class image quality' is going to fall short of my Canon 600/4L IS II, but how far short? If the IQ is decent (Tamron did a good job with the 24-70/2.8 VC), the AF doesn't suck, and the lens is priced decently, this will be a big benefit to many telephoto shooters.
I am expecting the optics to pale in comparison to Canon's more recent lenses, particularly the prime lenses.
Given the same vintage, primes beat zooms... you don't have to design for a range of focal length, you optimize for just one length.... and a lack of moving parts to zoom the lens means lighter and (hopefully) more robust.. I would love to see a Canon 400F5.6 for those who want reach and quality at an affordable price and still have a portable lens.
My experience with "longer yet affordable lenses" is:
Sigma 50-500... the worst of a bad breed
Tamron 200-500... almost as bad
Sigma 150-500.... better, but still not sharp
Canon 100-400... better, but lots of copy variation.
Sigma 120-400... best of the zooms, but BARELY ahead of the very outdated Canon 100-400
Canon 400F5.6... beats them all, but it is a 20 year old design with no IS
and interestingly enough, the 70-200 with teleconverter outresolves the works of them.....
All that said, Tamron might surprise me. Anyone remember thier 90mm macro lens from the adapt-all mount system? That was a classic lens that could compete with the best of them... perhaps they can do that again, but I doubt it, particularly on a big lens with no flourite elements...