Just to add some clarification... the AF is the issue I had with the 70-200mm. When I got it the AF was terrible, and I mean BAD! I sent it back and got a replacement and it was dead on accurate... on par with any Niikon glass I used. Another shooter I know uses the 30mm 1.4 and had the same issues on his D2xs, he sent it back and the replacement was perfect.
I would pick a comparable Sigma lens over a Nikon for a third the price anyday.