After shooting with the 100-400mm more, there is one thing I don't like with this lens. That is the position of the focus hold buttons on the lens. They are way to far forward. On my 400-800mm and the 200-600mm I owned previously, they were between the focus ring and zoom ring. On the 100-400mm they are out past the massive zoom ring. What that means is you can't easily access the focus ring, the focus hold buttons, the function ring, and the zoom ring from one hand position, even with a replacement lens foot that is longer. If I put my hand all the way forward on my Leofoto SF-06 foot I can access a focus hold button, the function ring and the zoom ring, but not the focus ring. Not a deal breaker as I don't use the control buttons a ton, but they are useful in certain situations. Having to reposition your hands to access full time DMF, for example, could mean loosing the object one is focusing on if it is small. Part of the issue is the massive size of the zoom ring, which limits how close the various controls can be.
IMHO, I think the lens would have been better with the function ring and control buttons between the focus and zoom rings. I think even putting the function ring out past the focus hold buttons would have been better. I guess since this is how they set up the 300mm, 400mm and 600mm, it is how they do it for GM lenses. The thing is those lenses have one fewer rings as there is obviously no zoom ring. Therefore everything is in reach. Those that have shot with those for a while will feel at home with the exception of the focus ring is now the zoom ring. If you are coming from the 200-600 or 400-800 it will take a while to get used to the positions of the controls.