I was surprised to see it in the hands of pro shooters. Of course they still had a 300 f2.8 or something bigger on the other camera but I never saw Mk I being used by the pros. I don't know about the 200-400 but this is really good and comparatively cheap . The AF works really well (used it on 5D mk III) and the colors and contrast are great.
I have both the 100-400II and the 200-400. To be honest, the use of my 200-400 is decreased a lot. For portability, that 100-400II is really great. When in the outside hiking, I mostly have my 1Dx with 16-35 f4, 24-70II and the 100-400II and a 1.4x extender. Handholding, the 100-400II is a great lens, even with the 1.4 extender on FF. For hiking once again the 100-400II has it advantage. What is the use of my 200-400 now? After buying the 600II really zero. I'm not convinced if I do want to keep that lens. There is almost one stop difference between the 100-400II and the 200-400, for the latter one. That can be important for sudden situations. The IQ of both lenses is great. Almost the same happened by Nikon with the new 80-400 which also performs extreme well and the 200-400 over there is also under pressure of this 80-400 lens.
Camera: 2x 1Dx, 5D3, 7D2, 7D, 300D | Prime:14 2.8Lii, 24 1.4Lii, 50 1.4, 85 1.2Lii, 100 2.8Lmacro, 300 2.8Lii, 600 4.0Lii | Zoom: 16-35 4.0L, 24-70 2.8Lii, 24-105 4.0L, 70-200 2.8Lii, 100-400 4.5-5.6 Lii, 200-400 1.4x 4.0L | TS-E 24 3.5Lii | Other: 1.4xIII, 2.0xIII, YN600EX-RT, YN560iii, Lee