I own the Sigma 10-20mm F/4-5.6. I would trade it in a new york second for the 11-16mm Tokina. The sigma works wonders for landscapes, but it is miserable for inside work if your camera cannot handle high ISOs. I would kill for the F/2.8. It also has fairly miserable distortion outside of the center..Id say..15% of the frame. Forget it if youre shooting anything with a face or straight lines and want to fill the frame at all. I hear this is almost completely eliminated with the Tokina. Something I would pay double the price for if need be. I am currently in the market for one of the Tokinas, but I need to get rid of the Sigma first.
That being said, sont get me wrong. The Sigma is an extremely capable piece of glass. It handles its own very well..you just have to know the lens' limitations. Ive shot many photos with it, some very successful. Most have to have the corners cropped out because of distortion, however. You just have to know what youre getting yourself into.
Mark