DXOMark has completed their review of the Sigma 18-200 f/3.5-6.3 DC Macro OS HSM lens for Canon. They've compared it to other superzooms from Canon itself as well as Tamron and found the Sigma to be the best performer. On top of it being the leader in its class, this new lens actually costs less than the lens it replaced.
“Sigma’s cosmetic revamp of the exterior and revision of the optical construction has resulted in a lens that not only looks classy but also produces results superior to rivals including those from Canon.
A reduction in weight and size, albeit only slight makes this all the more tempting for travel, while the enhancement in sharpness and optical performance generally at the longer focal lengths is particularly valued in zoom like this. Although this has led to a certain amount of compromise with the performance at the wide-end, retaining the modest price of its predecessor the Sigma easily impresses.”