Is the Nikon really worth 1000 bucks more?
The reason I want this lens is for low light action shots.
Depends on the person. I would not really call the Sigma 70-200 "pro-level" glass, it is more consumer oriented. It's not terrible, but it is not the best at anything either. Not the sharpest, not the clearest, not the one with the least distortion, not the best built, and not the one with VR... all those characteristics belong to the Nikkor.
My take really is "buy once and buy the BEST when it comes to lenses and lighting equipment and replace the camera body and accessories when needed".
I can always get the lower end glass faster/sooner, but as I've mentioned several times here... I would rather do without than settle for #2. If that means that I have to wait several months to save up more money, thats what I do.
The Nikkor 70-200 VR F/2.8 is the best lens in this range on the market for Nikon cameras... bar none. It is heavy and expensive but to me, worth every gram of weight and every penny that I paid for it. It's fast, clear and crisp and currently sits at the top of the heap. It has no competition... it *is* the competition.
I am not a Nikon leg-humper. When a 3rd party makes glass as good as or better than the OEM, I say it. I own more Sigma lenses than Nikkor (30mm F/1.4, 18-50 DC EX HSM, 105mm F/2.8 Macro, 15mm F/2.8 Fisheye, 10-20mm DC EX Ultra Wide and I may be missing 1 or 2 in that list), but when I want the best quality in the 70-200 range... the Nikkor is the one that is on my camera.