Beautiful day today at Lackford Lakes in Suffolk. Unfortunately, not too much to photograph and generally too far away. All lenses give good results on all bodies if the subject fills the frame. What we want to see in testing a lens is whether a cropped image is sharp. Here are some cropped images. They are all taken at 600mm, f/8, iso 640, hand held (as always) on a 5DIII. All images had minimal processing but had 0.9 pixels worth of USM at 100%. They are all 100% crops.
So, I must emphasize, I haven't chosen these images to show how great the lens is, show it at its best or act as Tamron's publicity agent. These are various crops to show what the lens is like, and you can make up your own mind.
My take: it's good enough for me in absolute terms - not just because it is is good value for money; I will use it at f/8 at 600mm as f/6.3 is somewhat soft when cropping; it is much better on the 5DIII than APS-C (just as is the 100-400); it is a pleasure to use hand held as it is nicely balanced and not too heavy; the AF is very consistent, but a little slow at 600mm; I haven't been able to test it much for BIF but my first impressions are that it is very similar to the 300/2.8 II + 2xTC - if anything it seemed to lock on better and not lose the subject.
In my opinion, it renders the 100-400 obsolete and Canon had better come up with something good to compete.
Notes: the Mallard was not over-cropped; the lapwing was very far away and is heavily cropped - I think the quality is very similar to what I get on the 300mm/2.8 + 2xTC; the flying swan was picked up very easily by the AF; the Yanks are clearly following me from up high.