OK, short and sweet: I previously owned a 5D and 17-40mm, and for my first FF it was a delight when taking 20-second exposures on a tripod outdoors, although low-light indoor images were not so grand. Then I got the 5D2 and the 17-40m all of a sudden became the weak point. The 5D2's sensor out-resolved the 17-40mm noticably, I was fairly disappointed with the results.
I sold the 17-40mm and got the 16-35mm, and WOW... my whole world opened right up again. The 2.8 already gets you a brighter viewfinder, but the image quality in the 16-35mm is the juicy, rich satisfying tone that I get from my 24-70mm. And indoor low-light images are WONDERFUL. OK, 16mm image have cruddy edges, all Canon wide zooms tend to suffer here. But to be honest, once I correct perspective in Photoshop I have to crop a bit off each edge afterwards ANYWAY, so I end up with superb results. Just like trimming the fat off yor steak, no big deal. With 16mm, believe me, there will be image available to trim.
The 16-35mm is superb image quality, right up there with my 100mm macro, 70-200mm/2.8 and 24-70mm. Couldn't be happier.