it is sad that no one who uses this lens with Nikon has responded yet to your question. I use this lens but with a canon 40D. I am very happy with the lens. It may be the case that you will not get autofucus with a 1.4 converter. However the lens has good sharpness when wide open. It is for a 500 lens easy to carry around compared to the canon or Nikon f4 lens.
I use this lens on a D300. Haven't tried any of the Nikon long lens but the sharpness and detail on it are fantastic as is AF. I use it with a kenko 1.4x and although af does slow a little its still well usuable.
Try before you buy! Mine is back in Japan as there was something wrong with the optics. Really sharp at F6.3+ but unusable wide open.
Sigma have loaned me a courtesy lens and its perfect!