Summary: On canon full frame at first glance a must have lens. Everybody seems to like it, well....... I don`t and i am not overly critical too. It seems that internet talk an clever canon marketing give this thing an much better rating than it deserves.
Pros: 17-40 range on fullframe, Looks nice
Cons: Low color contrast, not sharp, much plastic and PRICE
Excerpt: This is what I use for interiors and landscape. Best if focused manually. I have a 5DIII and use magic lantern which gives you focus peaking (plus a lot more) on the 5DIII and that makes critical focus easy to achieve. The lens produces very good colors and detail if used at f/ or f/11 especially.
Excerpt: I use this lens as an all-purpose lens, since I have an APS-C body (equivalent FoV of 25.2-64mm). It is very light, so makes a perfect walk-around lens. This is my first L-series lens and it feels great! It feels solid, the zoom and focus rings move very smoothly.
Pros: Consistent Output, Fast / accurate auto-focus, Lightweight, Strong Construction
Excerpt: I've owned both the 16-35/2.8 and this lens. If you don't need the extra aperture of the 16-35, the 17-40 is a much better value. I use it on a full-frame camera (1D-X) and couldn't be more pleased with it. I use it primarily for sports (Basketball) and landscapes.