The Canon EF 16-35mm F/4L IS USM is the poor step child of the infamous Canon EF-16-35 f/2.8L. Reasons I have this lens istead of the faster 2.8 version. First, the "IS", especially since I wanted to use this as a landscape lens. Second, it was about half the price of the 2.8 version. This lens has been in my bag since 2014. No regrets.
The lens is well made (its an "L" lens), feels wonderful to use, and as expected, is really fast to focus. The images are super sharp throughout the range. At the wide end (16mm), it gets a bit wonky around the edges, like most super-wide angle lenses. This lens produces wonderful color and generally is a lot of fun to use. For how I use it, i don't feel that I miss anythign from the lost stops of light. The "IS" covers a lot of ground. I am not a "super-wide" angle guy... I rarely go wider than 35mm. But, once in a while, I get the itch. This lens gets it done.