No more samples yet, but I gave the lens another workout today at the British Wildlife Centre (BWC). Leaving the 100-400L at home, I used it almost exclusively, with and without the Sigma 2x teleconverter.
Of course, this gets you up to 600mm zoom.
The following will re-cover some old ground, so I may be repeating myself a bit.
Negative 1: I knew this already, but if you need to go wider with a single lens, adding/removing the teleconverter in the field is NOT fun. In the end, to save time, I found myself stuffing the TC into my pocket without caps when not in use so I can switch it more quickly. Overall, in this application, I think the 100-400L would have covered 80%+ of the shots, and cropping would have adequately covered the remaining 20%.
Negative 2: It attracts people. The BWC is quite a camera magnet, but on this day I had the (physically) biggest lens. I got asked on several occasions what it was.
Negative 3: It is not parfocal with focal length. e.g. if you focus at one focal length, then change focal length, the focus shifts and you need to refocus. This is VERY annoying, because any zooming to track a subject will throw off the AF. For comparison, the 100-400L does not noticeably suffer from this. The 70-300L and 35-350L both do.
Negative 4: You can get 600mm... but it's f/5.6 then. And you still need to stop down one unless you like the soft look. 600mm at f/8 = high ISO at any sane shutter speed.
Negative 5: The hood still sucks. And so does the lens cap.
Negative 6: The minimum focus distance is rather massive...
Negative 7: It still weighs a lot...
Positive 1: 600mm for when you do need it.
Positive 2: f/2.8... ok, I've never liked f/2.8 zooms, but with more distant bigger subjects where adequate DoF is not limiting, I did play a little with seeing how bokehlicious it is. Seems good, but not perfect.
Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L, 100L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.