I replaced my non-OS Sigma 17-70mm for the Canon 15-85mm as I wanted the extra width and length as a single walk-about lens. One thing I would say about the Canon, while it is a sharp, well built lens there is noticeable vignetting when used wide open, so if you are shooting jpgs in your 450D then I would advise against buying it. The vignetting is cured if you shoot RAW and process in Canon's DPP software, or if you have a newer model with the in-camera correction. I don't find the IS of any use on this lens though.