Here's my first comment after ~7 hours of shooting the 645Z at the Calgary Stampede last night, and just starting to look at the files this morning. If you're able to afford it but really don't want to spend $9K USD on this camera, then stop looking at it, talking about it, or thinking about it. Put it out of your mind right now. Otherwise, get ready to make a purchase.
After 3.5 years of heavy use, I have still loved shooting my 645D's as much as I did at the beginning. As I've said elsewhere, the files were and are gorgeous, and have never left me wanting for anything when I was able to work within the camera's constraints. But the 645Z is really a much more capable photography making tool than I think can be appreciated just by looking at the upgraded specs on paper. In spite of the level of expectation I reasonably had based on a ton of experience with the D, the Z is already impressing the heck out of me after a single day working with it. Any doubts I may have had about getting this camera are history.
Ming Thein's review comments are right on -- the 645Z is going to substantially increase of the envelope of usability of the 645 system. Likewise, the conclusion of the LuLa first impressions review -- "It's a world-class image-maker in every regard, and the fullest realization of the dream of a 35mm quality MF camera." The LuLa phrase "35mm quality MF camera" should be understood to refer to shootability, not IQ. I was usually conscious of working within some constraints with the 645D; most of that sense is gone with the 645Z. The Z will still reward every scrap of shooting discipline that you want to use, but it doesn't force it on you any more.
It's going to take me some time to get into the details, including posting some images and making some DNG's available. But one of the few big questions I really had, which was the impact of CMOS vs. CCD on the "look", I think I can see is going to be a non-issue. Tone and colour look great to me on initial review. It's quite possible that very close inspection will bring to light some differences, but from what I'm seeing right now I think they'll be just that -- differences, not losses. And probably minor ones at that, at least for my type of work. I don't have head-to-head comparison files yet, but I did shoot the Stampede a week ago with the D. Images from last night with the Z taken in similar settings compare very favourably. I was able to use the Z in a much wider range of light and circumstances, and overall using the Z for this type of photography was a much more smooth, fluid and seamless experience.
So... big thumbs up so far. I'll post some images as soon as I can.