I have posted the test photos from my rooftop test of the Canon 17-85 EFS vs the Sigma 18-200, in the same focal length range. I found my Sigm,a to be a tad too soft on occassions, and not always focusing accurately. I intend to get the Canon 70-200 F4.0 L to complete the long end range of the Sigma.
As expected, the Canon is sharper, with better contrast. I tried to shoot both wide open, but accidently shot all of the Canon at 5.6. The most noticable difference for me are the blacks; they are much blacker with the Canon. As I stopped down, both were almost equally sharp; Canon a slight edge, but the contrast remained better on the Canon. I shot everything with the all neutral Parameter 2. Everything was shot with a tripod, and the center focus point was the same for all of the shots. The Sigma appeared to backfocus a bit at the wide end.
All of the shots are posted in full, no cropping or PP.