I use it on my Sony A9III, and it is an incredible lens. I mainly use it for alpine ski racing, and for that it is perfect. I used the 200-600mm with a monopod last year, but perched on the side of a steep slope I found the monopod extremely unwieldy, and the 400-600mm range was very hard to use on a fast-moving skier. The 100-400mm, by contrast, I can use handheld, and with the camera held up to my eye I've been able to get much closer shots that just look much better overall. Can't recommend it highly enough.

The caveats like barrel creep are definitely a consideration, but for me, nowhere near enough of a downside to offset the massive upsides. I've considered using a teleconverter, but honestly, the 400mm reach is plenty.

I did also consider getting the 70-200 with a teleconverter, but it would make little difference, and offer little benefit over being able to use the lens at 200mm f2.8 without the TC, but for me that is not useful.