Made the mistake of upgrading to the 17-85mm when I purchased my 20D without first checking out reviews on the lens. After shooting for a week, I am terribly disappointed. CA and distortion is so bad at 17mm that it renders the lens worthless for landscapes of general outdoor shooting. Some CA is still visible at 70mm and is obviously more apparent at the edges when capturing higher contrast images.
I actually had my 20D swapped for a new one thinking that it was the chip on the camera that was to blame. Image is very soft at 17 to 35mm. That makes it worthless for even amateur landscape photography. Probably acceptble to use for general purpose photography when used in the telephoto mode.
The only redeeming feature of this lens is the IS. Despite the useful focal range, this lens is a major disappointment. I am now looking for something else to replace it.