How is it compared with Canon 18-200mm?
I own both, I would say sigma is much better in terms of weight and size and slightly better in edge sharpness. However, the Canon 18-200 zooms smoother and a bit faster. Also, the Canon version seems more reliable to focus, I have more keepers from Canon lens.
One thing I noticed is that the sigma lens has worse light transmission efficiency. meaning even at the same exposure and f-stop, the image coming out from canon is about 2/3 stop brighter from my observation.
hope it helps.