Well it arrived this morning and I have done a couple of tests.
The first picture was taken indoors in pretty dim light with the 24-105 lens. Time exposure 1/6 sec handheld with and without the IS and cropped to 100%. Both straight out of the camera except for cropping.
The second test was comparing the Canon lens with the Sigma 18-200.
All are f5.6, camera on a tripod. The first in the Canon on 105mm, the second is the Sigma on 105mm and the third is with the Sigma on 200mm but resized to match the others.
As you can imagine I am well pleased with these results and I don't even lose out on the extra reach of the Sigma.
Dave