I tried this lens, as various good reviews (and the price!) were good - however, it kept locking up on my D100, as mentioned elsewhere. A firmware upgrade from Sigma would have fixed it, however I needed a similar lens for a theatre performance quite urgently. I bought the Nikon 70-200mm VR and it is AMAZING! Excellent performance. Even quieter than the Sigma, it doesn't lock up (of course) and the VR is truly phenomenal, treated with respect (ie don't go out of your way to wobble!).
I know the Nikon costs an arm and a leg, but if you're talking about a result against no result (ie pro use), then it's no contest.
Sigma OK, Nikon GREAT!