I din't need a 600mm lens and even if i did i would not consider the tamron lens, because tamron is not willing to equip their lenses in ef-version with matching zoomring anf focus ring rotation direction. As long as they make their lenses nikon style only Which causes lost shots for
canon users me, i will not buy from them. I liked the optical quality of the tamron 17-50/2.8, but sold it after a few months for this reason and got the canon 17-55 instead. I also trll the tamron people at any tradeshow i visit, that their products are seriously flawed.
I corrected your post
BTW, I got mine today!
First of all, the build quality feels great to me. The feeling is certainly not inferior to my 24-70/4 L.
AI Servo on 5D3 works on ALL points, also the side ones. AF seems fine even now: 1h before dark, very grey day, shooting grey pidgeons. It's not as blazing fast as a f/4 or brighter L lens, but I would say that it should cut the cake.
I can't comment on tracking because of my (poor) skill. For me, like many others, this is the first 600mm experience.