my 16-35 stays on my 5DII almost all the time. I love having the 16mm to 28mm range all covered by one lens. the main thing is, I rarely am shooting people, so the distortion is not an issue. if I enjoying/was paid to photograph people, I probably wouldn't use it so much. most folks I know find it bizarre that my walk-around is an UW, but it works for the way I see things - I can't stand the "50mm is the human field of view" thing, my field of view definitely goes way wider than that.
at some point I'd like to upgrade to 24 f/1.4 as an all purpose walk-around, but I have a tough time justifying the overlap in focal length for the time being (also have 24-70 L).