Bought this lens 2 years ago and never was really happy.
The middle, as expected, is sharp, BUT on a fullframe body like the 5dMkII you quickly see the not so impressive corners and edges: they are soft, absolutely not sharp, even going at f8, where every lens should shine.
I accepted it or used it only on the 1D, being a crop and so cutting of the worst.
After trying my first alternative, non original gear in my curiosity I found the Tokina 16-28/2,8 and:
it beats the Canon easily.
I´m very sorry for Canon and I`m no friend of writing something bad, but the 16-35/2,8II was a lens I avoided, because of getting a headache looking at the pictures.
(Sure, 28mm on the long end is less then the Canon, but before reaching this the IQ is far superior).
I sold it.
End of the story.
P.S.: I`m quite happy with all my other Canon- Lenses!