Just visited the Sigma HQ today, allowing me to ask a few intelligent questions while collecting some brochures for every of the 40 lenses offered by Sigma. First glance, the image work found on each catalog wasn't impressive. Either they've got amateurs to shoot for them or we are talking about shabby lens performance. I am not sure which. Maybe, they are not using the Nikon D90 body with the lens...but overall Sigma images lack that punch Nikon photographers are familiar with.
The only lens or two that shows potential is the Sigma 30mm F/1.4 and er...sorry, no other. The rest suffers from chromatic errors, and produce yellow-tinted, lackluster images (as seen on the Sigma digital gallery and brochures). O, they can take the pictures alright, but you can almost forget about showing the photos to your friends or family. Its that bad...
Now, the Sigma 30mm F/1.4 is like the beacon of light in the sea of darkness for Sigma. Its a different animal altogether. It shoots brilliantly for the price it commands at $400. All in all, Sigma is 12 years into the photography game, Nikkor, over 75. Obviously, there's a gap.
My next attempt is to fix one on and shoot with my Nikon D90. That should be more conclusive!