I have it, had the Version 1 as well, I'm not that keen on the Lens, seldom use it anymore except for Underwater Photography where the Corner distortion no longer presents an issue.
Not as Sharp as other WA L Lenses such as the 24-70f/2.8 L II (Have never tried the 17-40), it's not a cheap Lens, so for that reason alone it's a disappointment to me, What I've done in the Last 12 Months is Buy a couple of Zeiss Lenses which I now use instead of the 16-35, Currently I own the Zeiss Distagon 15f/2.8 & 25f/2, for sharpness at these Ranges Canon have nothing to compare, at all, But, this isn't the cheapest way to get WA sharpness, but it does get you the near perfect WA Lenses for any occasion, with the only downside being No Auto Focus, which in Landscape is generally not an issue as you mostly Focus in Live View.
I might suggest getting a Prime WA like the Zeiss 15 then later when you have the Money, go to the 24-70f/2.8 II, two amazing Lenses.
Someone in an earlier Post suggested Renting first, if that's an option I would say this is always excellent advise, then you can Try before you buy, win win.