I had the 17-40 for ten years and it was a great lens, used almost daily. Light, small and sharp as anything else from around f/5.6. Another plus for the 17-40 is that it takes your 77mm filters. The 16-35 f/2.8II is 82mm. Just something to consider in terms of additional expenses.
A month ago I switched to the bigger, heavier, 82mm filtered, more expensive 16-35 f/2.8II and to be honest, I'm not completely sure why. It certainly has commendable centre sharpness at f/2.8, (forget the edges...) that tiny extra 1mm of width and looks shiney and new, but between f/5.6 and f/11 the sharpness is line ball with the 17-40.
On a more subtle level though, there is a quality about the 16-35 f/2.8II vs the 17-40 f/4 that has nothing to do with sharpness, CA or any of the technical measurable stuff...the images just look nicer.
I hope that a new 16-35 f/2.8III is in development; one that will bring the awesome qualities of the new 24-70 f/2.8II to the UWA zoom table. If Canon is in fact shipping a 40+ mp DSLR anytime soon, that camera will punish the current 16-35 f/2.8II. A simultaneous announcement? We'll have to wait and see.
If your budget doesn't stretch to the 16-35 f/2.8II, the 17-40 f/4 will be sure to satisfy, so long as you don't expect stellar wide open performance. It's mushy wide open. But even one click down and it comes alive.
edit: BTW, don't be tempted by a pre-owned 16-35 f/2.8 series I. While there may be OK copies around, they're mostly rubbish and certain to disappoint.