Got this lens as I got tired of the kit lens. On my 300D the kit lens was rather ok but the kit lens I got with my 30D was much worse so I decided to buy this lens.
I have considered many other lenses before like the sigma 18-50, tamron 19-35, canon 17-40, 17-85 etc etc. but none of them really got me excited. The sigma/tamron etc mostly because of the noisy autofocus and the 17-40 because of it's limited range and 17-85 because it didn't offer that much of an improvement over my old kit lens. The sigma was the clear winner in build quality though..
I have a sigma 70-200 as well and don't really miss anything in the 55-70 range. But the sigma is much better built.
All in all though I'm very satified with the features and image quality of this lens .. so highly recommended if you're not too worried about ef-s..