In general, the sensitivity of the focusing (crosshair) sensor on a Nikon camera is usually effective down to f5.6, any lens slower than that then you will experience difficulty in focusing. As the Sigma is f6.3 at the tele end, it does exhibit some hunting. I don't own one, but had the chance to play with one on a Nikon F100. Focusing is very quiet due to HSM, but at the tele end, focusing is a little on the slow side, even on a sunny mid-day. The good thing is that you will get 750mm versus 600mm w/ the Nikon. Don't have any experience with the Nikon VR though.
George Oei Madison, WI "Dude, you're getting a Nikkor !!!"