Amazingly, the reviews I've come across rate the Bigma higher than the Nikon 80-400VR. I found this to be somewhat surprising but then again I've found alot of positive comments about Sigma's bigger lenses....namely the 50-500 and the 300-800.
They can't beat the main manufacturers when it comes to the shorter lenses, but maybe Sigma has something right when it comes to their long zooms. I know that when it comes to primes 600 and under and any zooms 300mm and under that Nikon glass (or Canon glass for that matter) often cannot be beat. But the one review of the Sigma 50-500 I read actually placed it above the Nikon 80-400. Weird...but maybe not?