I've done a lot of research on DX super-wides these last couple of days (just ordered a tokina 11-16 2.8), and as far as i'm concerned the tokina 12-24 f4 is faster, better built, and cheaper than the sigma.
I just spoke with a friend that has a sigma 30 1.4 which he uses along with his Nikon 24mm f1.4 and D3s. He claims to handle his equipment delicately, so I asked how he likes the sigma and he responds, "it's my second copy." This is just one more case of a bad sigma copy to add to the long and growing list
With the Tokina 12-24 ($400) as an option, the sigma really shouldn't be up for consideration.