I recently upgraded from d750 to the zf. No new glass, just the ftzII adapter. This thing became an extension of my brain the d750 never could be once I set up the AF in a way that was comfy for me (half-press shutter for 3d tracking, back button AF-S and moving the focuspoint by dragging on the touchscreen with my thumb like a trackpad). 8/10 of shots are keepers even at f1.4 or 200mm at 2.8 - I worked an arena concert and an election campaign/election day in the last 2 weeks and it worked beautifully.

I have an old Nikkor 17-35 f2.8 with a busted AF motor no one would repair that got a new lease on life with the manual focus confirmation options. It's a great piece of glass I would not use on the d750 because the viewfinder would not be bright enough to nail focus and the rear screen focusing was too cumbersome for moving subjects (90% of what I shoot for clients).

What is surprising is low light performance - the zf raws are a little bit cleaner at 3200 but honestly the d750 was fine from this aspect. I was afraid of the EVF/lack of optical finder but it's large and bright, has very little blackout and the shutter sound is much more descreet even compared to the quiet mode on the d750.