I finally did the plunge and I am totally happy with the Z. I have a 645D, which I decided to keep as a backup. Also have a stable of lenses, too crazy to list.
Already after a the few times of use, I must have dinged the aluminum Pentax plate. Don't know yet whether I should get it replaced. Slightly less solid than the 645D. But more a sign of how much I like the camera and that I am not afraid of using it for anything. Perhaps having the backup helps. Perhaps I'll keep the ding as a beauty mark.
I like the files and don't see much of a difference to the D, even though I prefer the D files just ever so slightly, when shot from a tripod at 100ASA. But the ability to crank up sensitivity is out of this world. 800 and 1600 ASA settings are readily usable. Going up to 400 already reduced file quality when shooting with the D. The enormous sensitivity of the Z allows me to easily use of all of those nice telephoto ED lenses, 300, 400, 600, and 800, in 645 and 67 versions (300, 400). The detail in the files is amazing as well. You can easily crop and you'll be amazed how much is left. Also great to use all of my many adapted lenses. Speed doesn't seem to be a limit anymore.
I accidentally hit the movie recording button, and that was a revelation in itself. Medium format video. Unbelievable. The battery life is another plus.
Only when I downloaded the files and saw my memory quickly fill up I realized that there was a price to the Z...