I am in the process of moving from D850 + Zeiss to GFX. You've gotten good answers so far. I'll echo some of them, and add a few things:
1) Lens quality of 120mm and 250mm are on par or exceed Zeiss. I primarily used 100mm/2 and 135mm/2 on D850 for most of my work, typically at f/5.6.
2) Nothing compares to the focusing experience of those manual helicoids. Focus-by-wire is only average, not great on GFX100. X1d focus-by-wire was faster/better. I would like to see Fuji open up the option to configure the "focus curve" on lenses -- how fast they accelerate versus the motion encoded by the ring. This is a small usability issue.
3) GFX100 wheels and buttons are not especially ergonomic for me. However, they are acceptable. They are more than fine on a tripod.
4) GFX100 viewfinder with the angled extender thing are amazing. The flip out displays are not good enough in bright conditions in the field. That the viewfinder can move around so much makes it usable at all kinds of orientations. I look forward to a radio-based viewfinder one day that I can wear like a jeweler's loupe (or a pirate's eye patch!) and not have to bend over at all, however.
5) GFX has unintuitive switching between various basic modes and strange interaction with how you manipulate ISO/Av/Tv degrees of freedom. It is learnable but the least obvious of any camera I have used.
6) I don't like the aperture rings on my two lenses. My gold standard for aperture rings is Leica M. I prefer to just use the wheels for aperture instead, but (a) the rings get bumped a lot and lock the camera into a weird mode and (b) see #5. Oh well.
7) Focus stacking interface is obtuse and Japanese. I would prefer something more intuitive, like Phase's.
I would love in-camera auto stacking, like Phase. I think this is something Fuji could ship in firmware, if there is not an intellectual property issue. (I don't know.)
9) IBIS + in-lens stabilization is incredible. I have shot on modern Nikon IS lenses, but no "serious" work because they are not as good as the Zeiss. It's good enough that I would consider using it handheld for some "semi-serious" work. It still makes me laugh when I see a 1/20s 120mm shot sharp as a tack at 100 megapixels.
10) I like the dual-battery and USB-C on this camera. It's nice to carry one cable and a laptop and be good to go.
11) I normally don't shoot the D850 with a vertical grip, so this camera is bulkier for sure. It is not really any heavier.
12) There are a ton of special settings and custom configurations available. It is overwhelming and very Japanese. There are also arbitrary spots where, for some reason, you cannot make a specific change or override a certain button, even though there's seemingly nothing special or important about that button or change that should prevent it. I actually wouldn't mind having a nice app on my phone to deal with all the complexity, but I know that's a bridge too far for most camera companies at this point.
Like others who have posted, I think this is a special camera with a strong runway ahead. I'll probably stop using the D850s entirely when I add a second body as backup.
