As they are cheaper and have good quality, I now have an "arsenal" of Sigma lenses. In a couple of trips to NYC sold my "old" lenses and bought the Sigmas 70-200mm 2.8 OS, the 17-50mm 2.8 and this 10-20mm 3.5. Kept my Nikkor 35mm 1.8 though.
#1. "RE: Testing out the Sigma 10-20 with my D5100" In response to Reply # 0
Forgot to say that my plans to buy Nikkor lenses (and of course new Nikon bodies) are not vanished by this. This is only due to monetary restrictions and as I'm only shooting a DSLR for about a year, I have time to just get the equivalent in Nikkor lenses in near future.
My verdict for the Sigma 10-20mm: Not the sharpest lens around. Shooting at its maximum aperture of 3.5 produces not so sharp images. Stoping down the image gets better and better.
#2. "RE: Testing out the Sigma 10-20 with my D5100" In response to Reply # 0
Nice images. Thanks for sharing. The Sigma Pro quality lenses are better than the Nikkor kit lenses. I prefer the 50mm f/1.4 EX HSM over the equivalent Nikkor 50mm f/1.4G AF-S. Good Luck and Enjoy your Nikons!
#3. "RE: Testing out the Sigma 10-20 with my D5100" In response to Reply # 2
Right. Definitely the pro Sigmas are better than entry level Nikkors. (I liked the 18-55mm Nikkor kit lenses though, despite the fact I've already sold mine). There are Sigmas and Sigmas. As there are Nikkors and Nikkors.
But I think - in most cases - comparing same model and category lenses between the two brands (and also Tamron and Tokina), the Nikkors (and Canons) wins. Say comparing my Sigma 70-200 2.8 OS with the famous Nikkor equivalent. The thing is for a lot of people just starting out and not making money with photography the difference in quality does not justify the enormous difference in price.