I've been reading the dialog between Diglloyd and Ming-Thien and thought I would respond to the issues they raise relative to the Pentax 645z
You have to be aware of my situation.
1) 99% of my photography is on a tripod.
2) All of that is live view, manual focus, magnified view.
3)95% of my photography with the 645Z is done at f11, or sometimes f16
4) Although I don't sell a lot of prints, I do get requests occasionally for 6 foot wide prints - I need the most pixels I can carry.
5) I'm 64 and have a bad hip - I'm not going to be hiking more than half a mile (the distance into Jura Canyon) any time soon, so +- 5 lb. carrying weight isn't relevant.
6) 95% of the photographs I make take 5-20 minutes to set up, find exactly the right position, get the right height on the tripod etc. so a little extra time to set the camera itself is moot.
7) the few times I need a faster camera, I generally know it before I leave the house and can take a more appropriate camera. I'm debating whether to hang onto the 70D because it will take my remaining Canon 70-200, the 400, and my two ts-e lenses, the 24 and 90, or more likely wait for the new Canon. If it really is good, then I might get that, if not perhaps the price of the 5d3 will drop and I might be able to pick up a used body.
8) for travelling with my wife, the A6000 is so light and compact and handy and did such a good job in Newfoundland, it's going to work well for some time to come.
Yes, I did shoot those horses the other day, and the Pentax 645Z did just fine, hand held, with the 300 mm. lens, for making a 20X27 print.
Consider too, what I was looking for:
a) a camera that mounted lenses the right way - and it's not just the lenses but every rear lens cap too (perhaps you can forgive Nikon when you remember the Japanese drive on the left side of the road).
b) I needed a fully functional live view, at open aperture - only Canon has that.
c) I've come to live the tilting screen on the A7r and the 70D. Yes, I'd prefer both tilt and swing, but tilt covers 90% of the situations, not the 50% you'd expect, so it's a hell of a good start and remote viewing via my iPhone may solve the rest (I have a flu-card on order).
d) I didn't like the button and control layout on the A7r and much prefer the situation with the Pentax.
And don't forget, I had already acquired an almost full set of lenses for the Pentax three years ago when I came close to getting the 645D (at about $300 a lens) which can still be done.
So, with that background, my decision seems almost a no brainer - even though I can imagine some situations where the Nikon D810 might be a better choice given the wide range of great and fast lenses for it - the Sigma 35 and 50, the Zeiss 15 and 21 and wide variety of long lenses. Yet, the Nikon zooms aren't that great - the 24-70 is mediocre at 24, the 70-200 f4 good but not outstanding at 200, the 80-400 (new) good up to 300 mm. and fine in the centre at 400, but not for landscapes, great for wildlife though (that I don't generally shoot).
So,for me, for now and the next few years, the Pentax 645z makes good sense. Whether it would for you will depend on your needs, desires, and circumstances. I'm hoping this explanation might help you make your decision.