Grandmama is right......it definitely depends on what you shoot too, but if it were a straight fight between the new Nikon AFS 50mm and the Sigma, I'd take the Sigma anyday of the week and twice on Sundays.
The Nikon was very disappointing for me: I tried so many copies, and they all had terrible bokeh with badly smeared background even stopped down. It's sharp but its sharpness is harsh, and the bokeh is harsh too. I certainly would NEVER use it for portraits because it is not good at skin tones, and if I wanted a general purpose 50mm lens, I'd shell out for the Zeiss Makro 50mm F2 instead.
But I shoot mostly people between the focal lengths of 50-135, and the Sigma is wonderful for that, and it is darn good as a general purpose lens too. The first few shots I took with the lens were at home, and my wife was the subject, and let's just say that the only times I have been so impressed with a portrait lens off the bat like that were with Nikon's own 85 1.4 and 105 DC. And the Sigma 50 is NOT a dedicated portrait lens even!
If it were a general purpose/walkabout 50mm lens I wanted, I'd be hard pressed to choose between the Zeiss Makro F2 and the Sigma. But I shoot people mostly, even in street photography, and for that, it's not a contest, so I actually sold my Zeiss and got the Sigma instead. The Zeiss is a cold lens, the Sigma has a warmth and a rendering that's unique, and it's not something that can be replicated merely by changing your auto white balance settings.
Like Grandmama said, do get a good copy, but that goes for any lens purchase these days, even the new Nikon ones (did someone say the 35 1.8, the 50 1.4 and the 10-24???) since they are no longer exclusively made in Japan anymore and have a plasticky feel to them in much greater capacity than previous generations of Nikon lenses. I tested several Sigmas before I settled on one, but I do the same for ALL lenses, regardless of manufacturer. QC control ain't what it used to be, not even for Nikon.
Btw, the Sigma overexposes by about 1-2 stops depending on the lens, so you'll have to factor in exposure compensation when you shoot, but it is a very small price to pay

It's the first 50mm lens I would use to shoot a wedding.