After reading the review, I am a bit surprised by some of the findings so I wanted to comment on my real-world use. First of all, the lens is expensive, but not in relation to the unique shots you can take and in comparison to Canon's long lenses.
Very sharp in center, very good sharpness until the furthest corners; Low CA, manageable vignetting, great build quality, very good flare resistance; The shots at 11mm are very difficult to do well, but are amazing when they come together
Large; heavy; no simple way to use filters; expensive