I've been waiting for Roger's testing for a few weeks. It's finally here.http://www.lensrentals.com/blog/2014/07/canon-wide-angle-zoom-comparison
Quote: "I set a high bar for new Canon lenses. I expect them to be excellent and generally their recent releases have been. Since this was a wide-angle zoom, though, my expectations were lowered a bit. Canon has always struggled with wide-angle zooms. The 17-40 is a good, not great lens. The 16-35 f/2.8 II is better than the Mk I replaced, but I’d consider it, at best, adequate considering its price.
The 16-35 f/4 IS changes that. It’s a superb optic — as good as anything else available. Of course, a lot of people want an f/2.8 zoom. But for many, like me, f/4 with IS is just fine for wide-angle shooting."