Having recently acquired a D810 I personally found the camera quite light and a little nose heavy when a lens was fitted. The cost of a Nikon Grip is not cheap, so a careful decision must be made. Upside is a more balance camera body/ lens, far better battery life and if needed more frames /second.
I also had a D4 battery available which helped me decide.So next question is do you buy the genuine Nikon grip or third party? I went for the genuine one and after checking various sources on the internet which reduced the cost.
So did you bother with a Grip or are happy just to use the D8----- solo.
Camera, Lens and Tripod and a few other Bits