Virtual horizon D3 vs spirit level
My experience with the virtual horizon on my D3 while out shooting today:
I found that with a 17-35 attached to the D3 that there was a 1 to 2 degree error that the D3 was insensitive to. The virtual horizon did not even show a change when I adjusted the level using a hot shoe spirit level. This was on top of a cliff with about a 30 degree forward tilt. I would not call that a sharp angle. It is noted in the manual the "virtual horizon display is not accurate when the camera is tilted at a sharp angle forward or back".
It is grossly accurate if my camera is pointing directly at the horizon...but I don't shoot pictures with the horizon in the middle of the viewfinder. If I want to define the perpendicular from the horizontal I will use a hot shoe spirit level. If I lose my hot shoe spirit level, this Virtual horizon is a gross back up.