I'm currently using the 17-40mm on the 40D. Have you compared your images to others posted on sites like flickr? The 17-40mm won't be as sharp as the 70-200mm but I've been very happy with the landscape shots I've taken with it. You can take a look at some of my shots at


http://www.flickr.com/photos/digital-aberration/tags/ef1740f4/