in short.. you get the EF 100-400mm quality from 150-400mm on the tamron ....plus an additional 200mm with not so good resolution.
now the question is how good is VC and AF with this lens.
Indeed. It seems to lose ~25% of its peak resolving power (at 400mm) by the long end. I wonder how that compares to cropping the Canon 100-400L? Also, 600mm will have thinner DoF, meaning AF accuracy is more important. A higher shutter speed may also be required, meaning higher ISO.
Still, the new lens delivers surprisingly good resolution performance for the focal length and cost.
And that is the question I have also. I've put off buying a friend's 100-400 Canon lens because after comparing it to the 400mm 5.6 prime, I decided I really want the lens for the reach of 400mm. Not the flexibility of zoom. I have my 70-200 f4 IS that does well enough for me at this point in my learning curve. (yes, the f2.8 would be wonderful since many of my pics are low light... but the weight and cost both are a hindrance)
My guess is that the IS of this Tamron may be enough to sway me since I'm not going to be setting up in a blind and waiting for wildlife to find me. I'll be traipsing around and trying to find a stump/limb/trunk/rock to be my brace. BUT, the 400mm Canon would be considerably lighter if I can get similar results from a crop. I bit the bullet this past summer and moved up to the 5Diii on a good deal and LOVE the FF experience. I don't want to spend money on a crop body right now, it would be better spent on good glass.