One interesting comment that caught my attention in the Imaging Resource review was the claim by Nikon that the image quality of the D850 will be a full stop better than the D810 at higher ISOs.
"While they didn't attribute it to the BSI sensor, Nikon told us that the D850 should produce the same image quality (both JPEG and RAW) at twice the ISOs as the D810, a full-stop improvement. That is, the D850 at its top "native" ISO of 25,600 should deliver the same image quality as the D810 did at ISO 12,800."
Now, I very rarely use ISOs as high as 12,800. However, if that increased image quality also at applies to ISOs such as 400, 800, 1600 and 3200, I would consider a full stop of improvement at such ISOs to be a very appealing upgrade.
However, I imagine the improvement over the D810 will vary according to the ISO chosen. Perhaps it will be a maximum of one full stop better at very high ISO's, and perhaps only a 1/2 stop better at ISO 400 or 800 or 1600; but that's still a worthwhile improvement.
This camera's going to generate an enormous amount of discussion because of the multitude of significantly upgraded features, compared with the D800E and D810.