Richard Snow , Aug 11, 2011; 07:47 a.m.
I have the Nikon 50mm f/1.4 and have (briefly) shot the Sigma.
The Sigma produces more pleasing background blur than tha Nikon, (though neither produce what I consider extremely smooth bokeh) and the Sigma focuses a bit faster.
Both are a bit soft wide open, but are equally sharp once stopped down a bit.
The Nikon is a bit lighter and, although AF is a bit slower, it seems to be more precise when wide open...which seems to be Nikon's focus on all of their AF-S f/1.4 lenses. (pardon the pun).
Both are close enough price wise so there's not really a comparison there.
In the end I chose the Nikon...mostly because I've had bad experiences in the past good copies of Sigma lenses...but I see that complaint less and less these days and I don't usually even try Sigma lenses, but my friend insisted that I try their 50mm f/1.4 offering before I bought the Nikon.
Hope this helps