Don't judge the performance of the lens from the link posted by the OP. These images look terrible. This is not intended to antagonize the OP, either their copy is faulty or there is a serious technique issue. I have posted comparison images to the Sigma 10-20 on dpreview. The lens is exactly what you would expect it to be: better than the third parties I have owned in pretty much every respect. Not by a large margin, but if you start looking you see it.
I have not compared directly against the 12-24, but to me the lack of 10-12 mm is a deal-breaker
