This is probably the zoom lenses in Canon's Luxury series that I am at least impressed. Do not misunderstand me - it is an objective of high class with an appropriate range, wide vinvinkel (in full shape launcher), really good build quality, weather sealing, comfortable controls, excellent focusing distance, good brightness and a fast, quiet autofocus!
What is weighing down this lens is instead its varied acuity over zoomomgånget. Not only between different specimens, but also within the same specimen varies sharpness. I have access to micro-adjust the autofocus on my 5D Mark II, and have found that this lens is in need of about -10 in wide-angle mode and +4 in the telephoto position, and somewhere in the middle of this at focal lengths between the end positions. This is obviously not possible to solve with today's cameras, so you have to compromise and set an average value which is far from optimal.
It is certainly not uncommon for zoom lenses suffer from this variability, but on a lens in this price range, I had actually expected more. Somewhat surprisingly, its predecessor, the 28-70L, much smoother and better sharpness across omgånget.
Hope that Canon will be more successful successor ...