Have been weighing up between the 16-35mm f/2.8L and the 17-40 f/4L for the past 4 months, to use on my 5D Mk II.
My 24-105mm is just not wide enough for some landscapes or nightscape scenes, and is not fast at f/4. But great for general landscapes.
I wanted the 16-35mm initially because it was faster than the 17-40, and better for nightscapes. But it was also twice the price.
Since I got the Samyang 14mm f/2.8, it's now covered my need for a fast super wide lens for nightscapes, so it's helped make my decision to go with the 17-40mm f/4L.
Only 1mm longer than the 16-35 at the wide end, a little more reach at the zoom end, half the price, and no doubt a brilliant wide angle landscape lens on the 5D Mk II.
It's been on my wish list since getting my first DSLR (350D) many years ago. I can't wait to try it out on the 5D Mk II!