Kaeru

I rate this lens '10' ONLY for my second copy. My first copy would have been barely '5'.
I own 16-35 2.8L II but I rarely use 2.8-5.6 as I only shoot landscape and I want my gear to be as light as possible for backpacking. Also I'm not happy with 16-35's 88mm filter size since I cannot share the filter with my other lenses.
I was very skeptical of this lens due to mixed reviews but after I read the review on terragalleria.com with comments from well-regarded photographers supporting the comparison result, I decided to give it a try.
http://terragalleria.com/blog/2010/08/17/canon-wide-angle-zooms-comparisonreview-16-35f2-8-l-ii-v-17-40f4-l/

My first copy was horrible, nothing like the tested sample on terragalleria.com so I returned it and I got another copy, which was sharper but still the left bottom corner was pretty soft even at F11-F13. So I took it to Canon and asked them if they can fix it. Two weeks later, I got a call from Canon saying they "calibrated" it. I was really surprised with results from my tuned 17-40mm. It's like another lens. At F8, it defeats my 16-35 II and my friend's 16-35 II in terms of sharpness from center to corners.
However, 16-35 II remains sharper in corners at any aperture wider than F8. So if you need to shoot under low-light with hand-held, go for 16-35 II. If you use tripod in low-light, get 17-40, it's more flare resistant.