I have the D800 and the D810 so able to do a side by side comparison. Question is , is it worth upgrading to the D810? first of all it depends if one wants to and can afford it. Apart from that then yes there is a marked improvement between them, mainly due to no AA filter, or at least that is what I think. What I did find was having to be more careful with avoiding camera shake as the D810 produces sharper images. Also the shutter is a lot quieter than the D800 which now sounds like a hammer in comparison, not good in a required quiet environment. The D810 now has an additional "i" button on the back which shows a camera and access to functions with shortcuts so you don't have to go into the menu so often.
Holding the D810 is better as they have reverted back to the D300 design with a much deeper grip to the front of the camera, why they made it shallower on the D800 beats me ,maybe some designer who never held a camera is my guess.
I have tried the video side ,often not mentioned, but here again I have noticed an improvement and now quite usable apart from that still annoying focus "click" in sound . This really requires an extrnl mike mounted higher up than the standard hotshoe fitted versions, I manages to find extn pieces for this purpose. Now new "p" lenses should go some way to avoid this.
Apart from minor layout changes there should be no problem doing an upgrade handling wise, what I did like is the access port on the camera side now has individual covers far better, the D800 has just one
One problem I had and really annoyed me on the D800 was not being able to get the right colour red in the photo, no matter what I did to correct it. However It had to go back for repair when I dropped it and the camera must have been recalibrated as I no longer had this red problem. Don't know if it was just my camera or a common fault on the D800's in general.
Having said that the D800 is still a great camera and if one is happy with it then don't upgrade, if you are a camera must have latest person but the D850 is out of financial reach then the D810 is for you.
The battery pack fits both so no worries about getting a different one.
Just one other niggle with all Nikon cameras and that is the damn strap they supply with them. Not only narrow but twist up constantly and i am forever untwisting it each time i go to use a camera. So to this end I have bought a Peak design "slide" strap which is a lot better. Why on earth does Nikon sell expensive cameras and supply rubbish neck straps? beats me ,for the few extra pounds it would cost , crazy.
So am I glad I got the D810? yes. Will I get rid of my D800? definately not or my D300 for that matter, depreciation does make it worth while