Hmmm not sure if I would completely agree with Richard on all his points.. I did test one when I bought my 17-40mm F4 Canon. Now, on to image quality. This lens is quite sharp. I tested it against my Canon 17-40mm  this  was considerably sharper at f4  than the 10-20 at f/5.6. to 6.5 However, at f8 to f/11 the difference becomes minimal, and the 10-20 is almost as sharp as the Canon..  The edges are a tad softer at f/8-11, but you have to look really hard to see the difference. Sharpness decreases quickly beyond f/16, and f/32 is terrible!!  The lens vignettes quite strongly at all focal lengths, even some at f/8, but  admittedly this is easily corrected.

Flare resistance is surprisingly good, better than the Canon. Contrast is good, not excellent, and colour is very neutral. I would say they may have a problem with quality control, and to be fair I found a 2nd sample  I tried to be far superior... Needless to say this was both using my 10D on the 5D the Canon is superb and provides a better range.. If you ever intend to upgrade to either the full frame or higher resolutions that may be on offer in the future, you may need to think more.. Build quality is very good if you happen to like the DX finish.. I am sure it  not so good as the Canon, Nor does it have the weather proof sealing...  The lens cap is very annoying and did not fit properly I lost one cap, and the rear cap is also poor quality... However I have to admit, that if funds are at a premium and you need the range and angle this lens can provide on a 1.5 to 1.6 sensor there is nothing better than the Sigma, and it is a good deal better than the EFS Canon version..  

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt