For the photographer is basically everything D810 is minus 12 megapixels and some less critical pro features (eg D750 is limited to 1 / 4000s shutter speed and lack 14bit uncompressed RAW). For most needed them not.
Far more relevant is that the D750 is significantly smaller and lighter camera, without for that matter compromise too much with the build quality. Now that I had time to take the camera for a while, I realize truly value out of a neater house on the move, considering the lenses so it is enough in the bag.
The shutter sounds less at 810 and to silent sounds not at all. D750n compared to more fuss but I can live with.
I have had time to use the camera both at home and on two trips and I think that it works wonderfully. There is never any delays, it is always ready and always respond immediately to my commands. The menus are user-friendly and Nikon Vana from before where it's no problem to adjust the camera exactly as I want it to function keys etc. I basically need never take your eye from the viewfinder, all settings I need is just a thumb / index finger movement away.
Compared to my previous D5100 are lifting huge. It really is a completely different camera. I can do everything so much easier and so much faster. The viewfinder is large, clear and focused system is of a different world. If you miss the focus is most likely a handling error, I have found that keeping up with all but the fastest / most complicated trackings.
And the picture quality is great. Full frame is absolutely wonderful. Impressive dynamic range, you can pull up the shadows without substantial noise (assuming of course that you plates RAW) as the exposure is rarely a problem. With D5100an I never wanted to go over ISO 1600, which saw the hopeless out. Here I can go up to 4000 without problems, often even higher!
With the right hands behind the wheel keeps pictures pro class, like me from a simpler APS-C, you will be amazed at what pictures you take with the camera.
You can also get very nice video in 1080 / 60p, however, no personally I concern myself much about, but only tested on a few occasions.
Finally, I appreciate features such as tilting the screen (do not have to shit down pants legs!) And wifi, which enables connection to a mobile phone, for example, direct transfer and remote firing with live feeds. Practically!
My personal complaint is that the tilting screen feels a bit fragile D500ans device feels more solid. The buttons could also have had better sense, even here, D500 and 810 clearly better (more comfortable "click"). And the focus, cruel as it is through the viewfinder, is pretty tough and unreliable in the live-view. I would not recommend the camera to video geeks, for this reason, if you are not satisfied with the manual focus. The small focus-joystick as the D500 and the new Canon 5DMKIV have had not been wrong. But, but, now I have finished whining!
D810 if you are looking for optimal detail resolution for studio use, accurate image editing and cropfrihet.
D500 if you're shooting sports / action and birds.
D750 if you want to take the step to full size and professional quality without adding too much money, and still have a camera that is (relatively) compact and practical.
Can heartily recommend!