When I got my 5dII I purchased a 17-40 shortly thereafter because of the good price compared to the 16-35. I hardly used it, found the focal length okay, but I didn't care for the overall handling of the lens, and wasn't really satisfied with the pictures coming out of it. It wasn't just for landscapes as I see so many people state, it was also to get shots of entire rooms indoors, pictures that show all five kids in a room and the things they are doing instead of just one or two. I enjoyed the focal length, but almost gave up on it thinking I was expecting something that I just couldn't get and that I should just realize it's a landscape lens like everyone said.
Well, I rented a 16-35 II and my experience was completely different. I shot with the lens, enjoyed the results, and enjoyed using the lens. The experience of taking pictures to me is fun and exciting, and the 16-35 made it much more enjoyable. Very fast AF, great color rendition, etc. I sold the 17-40 I had purchased new and got a used 16-35 from someone here. Definitely cost a few hundred bucks to make that upgrade, but I'm so happy I did.