Being that I just registered, I can't set my signature, so I'll say I shoot a lot with a 5DM3 and 70-200 f2.8L II. I sometimes use a 1.4x III or even a 2x with it. (The latter is rare, of course ... but I've done it!) Having had the 100-400 II for just a little over a week...which isn't much, I know...I'd say:
1) I may never shoot with an extender on the 70-200 again. In my opinion (I shoot a lot of action shots with a lens like this) the slower aperture is traded for the better IS. I can get sharp photos. That's what's important to me.
2) Focus is fast. Given I'm used to the 70-200 ... I was expecting the 100-400 to feel slower focusing. It doesn't.
As for things I don't like, there aren't any. But, there are 2 minor nits to offer:
1) I don't always remove it, so I wish the tripod foot was a little wider and longer. Or maybe, just have a "squared" off back to the foot rather than the curve it has. As-is, it shifts the "tip point" more forward. The 5DM3 barely balances. It will not balance with a flash. In most shooting situations, it's fine. I'll get into the habit of removing the ring!
2) I do not understand the point of the zoom tightness ring. I do not see any drift in focal length with the lens pointed at the ground or straight up in the softest setting. In the tightest setting, the zoom is pretty much locked. The softest setting feels right to me. For now, it's a bit of extra cost for nothing. Maybe wear will make the zoom looser and I'll be glad it's there.
I will shoot a few comparisons as requested, but I don't expect anyone to change their minds based on them. Both lenses are amazing. The 100-400 is a better lens than a 70-200 with 2x converter. Not to mention, it also works well with the 1.4x, pushing the focal length to 560 -- which isn't attainable with the 70-200.