I personally do not like grips. The D800 fits my hand better without one. I seldom shoot vertical and when I do my shutter speed is high anyway. I do not like the extra weight or bulk. I recently shot over 3500 images at a MotoCross Event without depleting the first battery, and I can easily go over 1 1/2 hours of HD1080 Video.
Go ahead and get one if you want, you can always take it off.