This is my first L lens. I was dubious about buying the 17-40mm L as there seems to be variable reviews of this lens. I had sold my old 300d with the 18-55mm kit lens and needed something with a similar range. After using the new 17-40mm with my 550d body for two weeks, I can say that the images taken so far are outstandingly sharp. Makes my other non-L lenses seem a bit dull! Perhaps I was lucky and bought a good copy.
Obviously the max aperture of F4 is a bit limiting but I have other faster lenses of different focal lengths if I am working in poor light.