I took delivery of my new 100-400 last week and have been out a number of times shooting with it..... both with and without the 1.4xTC III on my 5D III.
I'm delighted with the performance of this new lens. I have owned a number of the v.1 offerings, and never had one that would deliver sharp images wide open. f/8 was my go-to aperture for keeper images, not to mention it was a very slow AF lens. I have also owned a number of 400 f/5.6L lenses. I still have one but it will likely go up for sale soon.
The new 100-400 is well balanced on a gripped 5D III and handles beautifully. I like the new forward location of the zoom ring and find it is really easy to use when resting the tripod foot on the heal of my left hand and extend my fingers forward to turn it.
The AF is both fast and accurate. Much faster than the previous version and in my opinion, at least as fast as the 400 f/5.6L. The lens is also much sharper than the previous versions I owned, and in my opinion may actually be a bit sharper than the 400 f/5.6L. I shoot this new lens wide open all the time. Perfect.
I have also used it with the 1.4xTC III on birds in flight and if I do my job of getting the focus point on the target, the lens focuses without hesitation and very accurately.
It now has a permanent home in my bag.