Good comparison here: http://www.grahamclarkphoto.com/canon-16-35mm-f4-review-hands-on-shootout-17-40/
I just sold my 17-40L and got a 16-35 f4 myself, because I do shoot interiors and events where the edge/corner resolution at wide-to-moderate apertures is important, and the IS is a very welcome addition for travel. My 17-40 was never perfectly centered, even after repair, so that exacerbated the sloppy edge/corner resolution on the left side of the frame and helped my decision. I'm extremely happy with the new lens. Biggest surprises so far:
1) Almost non-existent CA in the corners, even at f4. 17-40 wasn't bad, but this lens is in a different league. Very nice for architecture and landscapes with white features on the edges
2) IS is more useful than I thought. I can shoot at 1/4 s with my elbow on something and get very sharp shots. That's incredible.
3) As mentioned, distortion seems a bit better handled on the wide end
4) 16 vs 17mm is a bigger gain for me than losing 5mm on the long end. I have a 24-105, so there's still nice overlap
Good luck.