A few people have contacted me about using the Sigma 10-20mm on a Full frame Camera as I have used this combination on my Canon 1Ds mark ii. In order for the lens to work without any vignetting you have to use a KENKO 1.4 x tele-converter. I have the one with a blue dot on it which I think was the newer one. The resulting set-up leaves you with an equivalent of a 14-28mm lens on a Full Frame Camera. You will lose one stop of light but if you are using this for landscape shots then it´s no big deal.
I have four A3+ framed prints in my house and they look fine. Also a 70 x 50 cm framed print which looks great.
The reason I used the Sigma 10-20mm was that I had it for my Canon 500D and it was cheaper than paying 600 euros for a Canon 17-40mm L lens and I really didn´t want to get rid of the lens as I used it on the 500D quite a lot.
Here are some sample images. Remember they are saved for WEB so the real life resolution is better.