Summary: This lens seems to be really solid. I like the zoom range. The extremely wide angle of 16mm is necessary at times, but like all wide angles you get distortion which is the nature of the beast. So I really like 20mm because it reduces some of that. 35mm is my personal favorite probably because of the shallower depth of field. Love the 2.8! I really don't have any complaints except the price.
Summary: I chose this lens for my 7D over the 16-35 II and the much-loved EF-S 17-55. The 16-35 II is obviously way more expensive. I chose it over the 17-55 due to the build quality and the extra mm, I already have the 36-55 range covered at 2.8, and the fact that I found an amazing deal on it. Like other reviews I read stated, it is a bit soft at 2.8. And there is some vignetting in the corners, even on a crop, but it's nothing Lightroom can't fix. However, I love this lens.
Summary: Great wide angle lens, definite improvement over older EF 17-35 F2.8. All metal body, smooth, fast, quiet operation, better optics than 17-40. Great value compared to 16-35 F2.8L II. Of these four Canon wide angle lenses this one has the best cost/performance bang for the buck. If you want the best Canon has, then pop the extra bucks for the 16-35 II.
Summary: great lens. i got mine used and my only gripe about it is that it feels like there is a 'grain of sand' in the mechanism. it doesn't affect performance at all. i only notice it if i take the zoom to either extreme and then turn the focus ring further. if i keep the focus ring tuned to where the focus should be, i don't get that grainy sensation. i assume this is because i got the lens used and maybe something happened with the prior owner. but i still love the lens.
Summary: Personally, I think these "L" lenses are overrated and overpriced. Sure, they deliver better and sharper images and nice blurry backgrounds than the non "L" lenses shooting in well lighted conditions. But under low light conditions, you still need to make adjustments to get decent exposure with these "L" lenses, no difference from the regular non "L" lenses.
Summary: I'm a wedding photographer and own this lens (Version 1) plus the 24-70 f2.8 L. In my opinion, this lens is sharper than 24-70 in the middle part of the image. I can understand the vignetting for this kind of ultra wide angle lens but the flare is quite severe and the softening on the sides is not good. Fortunately, I focus mostly in the middle of the image, so I ignore the softening.
Conclusion: A zoom that I had more than 10 years. Despite its weakness in unsharp corners and poor flare performance, the zoom provides convinience and reliability when it comes to travel photography. My image gallery of 16-35/2.8 http://www.roentarre.com/Gallery.aspx?id=1&lid=6 http://www.roentarre.com/Gallery.aspx?id=1&lid=6&pid=2 http://www.roentarre.com/Gallery.aspx?id=1&lid=6&pid=3 http://www.roentarre.com/Gallery.aspx?
Conclusion: As it is always: you get what you pay for. And if you buy this lens, you know what you want and you get what you want. On a documentary, this is the lens I would use straight out of the bag. It is good for sports because it has the two key facts: F2.8 and a fast focus. The IQ @ F2.8 is good between 20-35 and OK at 16. Stopping down to F3.2 or F3.5 is absolutely usable for my needs and for the circumstances I am working under (bad light/no light).
Summary: I got this lens to go wider on a 20D. I wanted to go wider so I got a 5D. Started shooting events with this lens and I learned that the 580ex couldn't give me enough light in the dark. Used the lens during the day. Terrible problems with lensflare. The hood showed up on the edges of pictures (wide). Went back to the 24-70L as my primary lens. When this lens works and circumstances are right, the results are stunning.