When I purchased my 400mm f2.8S I had to sell one of my 3 cameras... the choices were the Z9, Z6II, or Z8. I love my Z9 thus I kept it, the Z6II fetched very little in trade so I kept it, but my dealer wanted a Z8 for their rental and offered me a very good price ($3400)... sold it. I then went to Bosque del Apache with my new lens mounted to the Z9 and the Z6II mounted to the 180-600. The Z6II did ok, but I missed the AF flexibility of the Z8... I missed it a lot... a whole lot more than my words implied. When I returned I contemplated buying another Z8 or used Z9 but I did not want to spend $3500 + on a camera... enter the Zf.
My same dealer gave me $1100 on my Z6II and I purchased the Zf for my 180-600 and landscape work.
Thoughts... the AF is maybe a hair less dynamic than the Z8, but it is way way way better than the Z6II/Z7II
At high frame rate you will get the slide show effect, thus the viewfinder experience is NOT as smooth as the Z8/Z9. If you've shot with a Z8 or Z9, you will notice a difference when shooting anything above 6fps. This is the price one pays for a camera that does not have a stacked sensor... but the camera is "only" $2000
AF tracking is more than fine. If you get the lens on the subject, it will behave like the Z8/Z9.
The built in grip is too small for a long lens, so be prepared to buy the SmallRig grip... it's nice, feels good, and has a built in Arca Swiss plate.
If you are used to a Z8/Z9/Z6/Z7, you can program the two command dials to behave like the other cameras... this is how I use my Zf. However, be prepared for dials that will be harder to find. The latter is especially true if you are wearing gloves. To me, this is the biggest irritation with the camera. I can live with the slide-show viewfinder experience, the small dials are a problem for when I shoot in the cold.
There are no "Custom Banks" or "User Banks."... I think that this might be the biggest oversight with the camera. The Zf has an Expeed 7 and thus more than enough processing power to save Custom Banks like my Z9.. Shame on Nikon for not giving us this one little thing that can speed up operation when shooting conditions change... this bugs me a lot!
Finally, the Zf has too few programmable buttons. Nikon should have included either a second function button on the front (yes it changes the styling) or added an AF On button in the back... One extra button and Custom Banks would make this camera a suitable back up to a Z8 or Z9.
So what does this mean for me... I'll be using the body until I can afford a Z8 or Z9 but will keep it for casual shooting and documenting travels. I also hope to learn how to do some videography and will use the Zf for this purpose. It's a good camera and represents a "bargain" in Nikon's line-up but it is an imperfect back-up for a wildlife photographer.
cheers,
bruce