I had the first edition (bought from a Hogger) and traded it for a mk2. I use it on my 7DII for hand held going after birds etc. (with a pistol grip). I have also used it on a 5DIV, 6D and 80D also. I haven't used it on my 90D yet.

It is fast focusing and gives excellent IQ. I often use it with a 1.4x III extender.

Two images, #1 without extender at 400 mm on my 5DIV and #2 with extender at 560 mm on my 7DII.

Also it focuses so close I often do insects etc with it instead of a macro.

It's a great lens. The zoom covers a great range, easily replacing any 70-200 f/4 zoom or any version of 70-300. For the newest EOS bodies, adding the 1.4x can autofocus at f/8 and multiple bodies can now use AF points across the entire frame, not just the center.

It's bigger and heavier than other options. I take those other options when I won't need the heft of the 100-400L. No one would complain about the image quality of the extended 1.4x, but if you have L-primes that take you above 400mm, the primes are sharper, even with the same 1.4x and 2x EF extenders.

My 2019 year-end review of my lens usage tends had the 100-400L II as a candidate for swapping out. Here's an image from some mid-2020 usage taking that swap-out idea off the table:

Ring-billed gull by Paul Sager, on Flickr
https://live.staticflickr.com/65535/49917456562_0007e5e342_h.jpg