When buying this lens, it's very hit or miss. I bought one yesterday, and while the optics were amazing, the AF was out of control! Focused about 4 ft too far away at medium distances, and about 3 inches too CLOSE at short distances. I've never seen anything like it before. Went back to my store this morning (which luckily is 5 minutes away) and switched to another copy. This one is MUCH better, I just had to dial in some fine tuning (to be expected with any brand lens), and everything was fine.
So, I would only buy this from a store with a good returns policy, and be prepared to try a few copies before you find a "hit". For the price and performance compared to Nikon's offering though, I'm happy. It's insane how sharp it is wide open. I also really like the strong vignetting, although this can of course also be seen as a negative.
As a plus, I definitely prefer the build "feel" of the Sigma to the Nikon model.