I've just picked up a second hand Canon 17-40mm f4.0 L series lens and taken it out into the new forest an to my sister's house as a trial run of the images it can take.
My first impressions are of just how different the L series lenses are to ones aimed at the amature market. The focusing on this lens is faster and quieter than the Canon 50mm I've been using, much faster and much quieter.
I can see why people say once you've used an L series you won't go back.
Interestingly, the zoom mechanism is backwards to the Sony lenses I have used, which will take some getting used to.