Royce, I couldn't agree more with what you said. My experiences with 645D, 645Z, and D800E have been more or less the same. The D800, D800E, and most likely D810 (judging from the specs) offer shooting options that are, quite frankly, downright weird. The bracketing spacing (exposure difference between shots) can be set only up to 1 EV, and you have to take 5 shots to cover 4EV, which gets quite annoying in fast-changing light conditions. Especially with 14.5 stops of DR the Sony sensor offers, I really don't see the point of having only 1 stop difference between shots. Customization of the bracketing order is also limited, at least compared to the D or Z, and this can easily lead to frustration in the field. The dedicated buttons on 645D have served me well over the years and they can make a huge difference in getting the shot.
Originally posted by Royce Howland I don't have any of the Nikon D8xx series and likely won't ever. But I did just recently pick up a Sony a7R to replace all my Canon 35mm kit that I'm now selling off (except for some choice lenses like the TS-E's to keep & use with the a7R). I expect to do some comparison work over the coming months, with the a7R vs. my older 645D's and my new 645Z. I'm sure each will prove to have its singular advantages.
From a shootability standpoint, I already know there are things about the a7R that annoy me in ways that the slow & heavy 645D never has done. The 645Z eliminates most of the downsides of the D's shooting experience (aside from sheer size & weight). I just plain enjoy shooting the Pentax in ways that I can't see ever being true with the Sony; I say that having no history with either company's cameras prior to the 645D and a7R. But we'll see, once I've had more time to become accustomed to the Sony's quirks.
As fantastic image-making tools, I do think these cameras all represent an embarrassment of riches.
It was 10 years ago that I bought a Canon 10D. Even having worked in IT for 30 years and knowing the pace of technology innovation, it's still amazing to think that we've come from the 10D to the 645Z or even the a7R in just one decade. While we haven't doubled hardware / technical qualities like resolution or dynamic range every 18 months as Moore's Law would seem to call for, it's still incredible what we can do with the current crop of cameras compared to that old 10D... which itself was no slouch for its time.