I shoot historic steam railroads as a high-end enthusiast. For the past 5 years, my primary camera was a D4, which I used for 90% of my photos. I also carry a D750, typically for long telephoto or super-wide angle shots. I need cameras with good low-light capability and a high burst rate as I shoot action and I never know what I will get for weather. I take a lot of night shots as well.
With both existing cameras 5 years old, I looked to the D850 as an upgrade, primarily to replace the D4 for everyday use. I purchased the grip because I need the burst rate, and my only extra expense was the grip itself, as I already had the EL-EN18 batteries and charger for the D4. After about 5 days of shooting in a variety of conditions, I am less than impressed. In good weather, the D850 produces great image quality and the burst rate is 9 fps as advertised. When the sun is out, it´s a great camera. But in poor light the performance is not nearly as good. I have my shutter set to focus-release and I use single-point AF (D9). Shooting black locomotives on a dark and rainy day, I found that the burst rate wasn´t nearly 9 fps and the camera shot in fits and starts, probably because the AF was not keeping up with the shutter. The D4 never missed a beat under those conditions. Also, the image quality was noticeably poorer than the D4 above 2000 ISO. I did not have much success getting clean images above 3200 ISO. A pro photographer friend on the same shoot indicated to me that on dark days, he wouldn´t even take his 850 out of the bag. He shot with a D4s exclusively in bad weather.
So now, as I prepare for another trip with unknown weather, I´m trying to decide what to take with me and quite honestly, I am thinking D4 and D750. The D850 seems like a $3,700 failed experiment to me, because both of my older bodies have better low-light performance. The 850 hasn´t earned a place in my bag. I probably should have put the investment toward a D6, when it becomes available.