I just recently purchased the 17-40L and its a gem for the price. On my XTI the lens performs just aswell as my Sigma 18-50EX except the sigma is f/2.8. The sharpness wideopen is absolutely amazing for a zoom IMO. On the 5d its an ultra wide lens (reason for purchasing) and it give a "fun" fisheye look at 17mm -2xmm due to barrel distortion.
When I first put my new lens on my XTI at the camera store I tested it wideopen hand held in real life walkaround useage. The eyelash detail when photographing my 2yr old daughter was very impressive. What impressed me the most was the colour/contrast of the photos straight out of the camera. Stopping my copy to f/4.5 the sharpness increased dramatically which is already extremely impressive at f/4.