I love my Sigma HSM. I had the first Nikon 80-200 F2.8 (push/pull zoom, which I do prefer over the turning left to right) but it wasn't as fast as the Sigma when it came to focusing. I was able to get a good deal from a friend on the Sigma so I bought it and sold the nikon.. Made money on the deal too.
I have the Sigma 70-200 2.8 EX HSM. The Sigma has HSM focussing which is the equivalent of the Nikon AF-S. I tried the Nikon 80-200 AF-S before buying the Sigma. I did not see any difference in the focussing speeds using the Sigma. The build quality was comparable.
I have not noticed any wierd color issues while using the Sigma for the last year. The images from the Sigma have been pretty sharp and colors have been pretty spot on.
I would recommend the Sigma if you are not planning to buy the Nikon 70-200 AF-S VR. It might be a better choice than the non-AFS Nikon due to the focussing speed.
OTOH, if you think you don't need the focussing speed, then you have to decide if the extra $250 would be worth going for the Nikon. Nikon probably will have better resale value but if you buy the Sigma used, you probably won't loose any money if you decide to sell it.