I finally got the 100-400 a couple of weeks ago.
I very much like the mechanical construction, the zoom requires some force to apply but not as much as the 200-500, so the 100-400 is faster to use but still more work than, e.g., 70-200 to zoom from end to end.
The tripod mount feels sturdy and rotates smoothly. I can mount a plate with two screws and there is no risk of slipping or unintended rotation.
The zooming, as Nikon advertised, doesn't cause much of a center of gravity shift so on my small Gitzo fluid head, no rebalancing is needed after zooming. This is great and one of the reasons I felt I could get this lens.
It doesn't have the kind of "almost sharp" appearance of the 80-400 at slow speeds (well, at the time I didn't have access to electronic shutter, so that can play a role), and even the maximum aperture can be used safely.
Bokeh is ok but it's not as nice as a non-PF prime. There is some swirl in the outer areas of the frame. I think it's "intermediate" in the quality of out-of-focus rendering overall.
So far my main source of disappointment is focus at near distances. The lens focuses really close, but autofocus for near subjects with the Z8 has been ... well, poor. Manual focus is frequently needed. I have shot with the lens in bright daylight (though this is relative, we are talking about November in Finland here ;-)) as well as in the pine forest floor in low light, and a lot of the time my ISO is above 12800, alas. However, this situation will improve it's just the worst time of the year for light levels before the snow falls and changes the landscape.
I post one example that I took yesterday. Z8, 100-400mm at 400mm f/5.6, 1/500s, ISO 11400. A slight crop has been made, maybe 1.2x. I think it's funny to think that it's the sun which creates those variations in light and this is around 2 pm in the afternoon, but still, ISO 11400!!! Welcome to Finland ... The picture was processed in NX Studio with noise reduction set to Normal. I think Nikon should rethink their high ISO noise reduction algorithm for the highest ISOs it's not the best. But at this size it's not too bad. My license for DXO unfortunately doesn't cover the Z8 so I have to think about upgrading.
Anyway, I'm wondering if there are any tips for how to deal with the autofocus when photographing small birds. I have tried 9-point dynamic area and small, large and custom wide area with animal detection and it isn't too reliable for blackbirds in these conditions. The found subject bounces around a lot. If it loses focus it can lose it so badly it stops trying. Is this the bright new mirrorless future?