I know this topic has been beaten to death regarding this lense vs the 17-40, so I'm not going to ask that.
However, I am going to say this: From what I've read, the only reason to get the 16-35 is for the extra stop which is useful for low light, indoor shooting. But I've also read that f/2.8 still isn't really good enough; and that if I really want to shoot indoors in low light I need a prime (24L or 34L).
Any insight on this lense's performance indoors? Churches, museums,etc? That would be the only reason I would purchase this over the 17-40 so if it perfoms "better than the 17-40 indoors, but still not great" then I'll have that $700 bucks and put it towards a 34L.