Dave: The Nikkor has more barrel distortion at 12 mm than the Sigma, which gets to me, as I'm rather sensitive to distortion for some reason (which is why I can't stand fisheye pics, but anyway

). The big plus with the Nikkor is that it takes filters, so you can get that great CPL look. I have the Sigma and it's great, I sometimes wish I
also had the Nikkor, but only for the filter ability. I wouldn't say one is better than the other overall, they both have their strong and weak points.
I think someone on here has the Tokina-flavoured one don't they? I'm very interested in the Sigma 10-20 which takes 77 mm filters (nice), will have to see how it turns out with respect to distortion.