I don’t see how you manage to get the lens to vignette on a cropped sensor, but for photographers used to fullframe (film or digital), the vignetting is about the same as any other wide angle zoom lens, so it should hardly be visible on a 20D. Besides, it is easily corrected in ACR or some other software if it bothers you too much. With my copy I don’t have any serious problems with flare either, but distortion is definitely more than normal on this zoom lens. The zoom range is convenient and the sharpness of the lens is enough to be able to live with the heavy distortion, especially since it also is easily corrected in software. However, it’s not very wide on a 20D, but you already know that...