Thanks, all, for the thoughtful comments. I am, indeed, considering the 645Z in light of my own shooting priorities (landscapes, shooting both a la Weston, within a 50 feet of the car, and also on longer hikes). For work on foot, I have the Sony a7r but when I don't have to go far would appreciate the option of a larger format for both intrinsic IQ and for larger printing. (I also like to crop to a square format and the 645Z aspect ratio lends itself more to that.) I didn't seriously consider earlier MF solutions because the challenge of focussing in the field, but with its CMOS sensor the 645Z now offers excellent live view at relatively reasonable cost and with other strong advantages for those primarily interested in field work. I've rented the 645Z with its 55mm 'kit lens'. I liked the IQ very much, found the weight not that objectionable for a short jaunt, and generally liked the ergonomics.
The two aspects of the 645Z that had given me pause were:
(1) the lenses: it appears the system isn't succesful enough in the market to attract other lens manufacturers and so one must rely on Pentax. The copy of the 55mm I rented was disappointing; below the quality of, for example, the Sony FE 55mm prime beyond the center of the frame. I read elsewhere that the FA645 25mm is similarly disappointing compared to the Zeiss 21mm f1.4 (another lens I use via adapter on the a7r). On the other hand, I have read good reviews of the 28-45mm zoom and the 90mm (regrettably, neither of which I can easily rent at my current location overseas) and these are the lenses I would likely use the most. As I probably shoot 80% of my work at base ISO between f8 and f14, I am guessing from what I read elsewhere on this and other forums that some of Pentax's more reasonably priced legacy lenses might serve well also. Any further comments on that would be much appreciated.
(2) the lack of an EVF: I appreciate that this is probably still a minority opinion, but I really like that the a7r's EVF has eliminated for me the need to focus while fiddling with a Hoodman loupe. Having said that, I found the 645Z's titling screen at least made focussing more convenient than with fixed screens of cameras such as the Nikon 800 series, with which I also have a fair amount of experience.
It was with those thoughts in mind that I was seeking the experience of those of you who have access to these or similar systems to see if the 645Z's advantages are significant enough to outweigh its limitations over the longer term. It sounds like that's the case, so, again, thanks for the great feedback.
Chris