I just picked up the Canon 16-35 F4 IS today, and am quite happy with it so far. It is roughly the same size as the 16-35 F2.8 but a bit larger diameter. I do mostly architecture and some group shots with the 17-40 or 16-35 F2.8, and this seems to be a better replacement for both.
Sharpness (my copy):
The new 16-35 sharpness is even across the whole frame indoors at F4, and at infinity the last 1/8 of the frame (Left & Right) are a slightly soft wide open till about F8. At F11 sharpness is about the same as F4. F16 is noticeably softer than F4. F22 is poor. F5.6 and F8 are best.
The corners are darker at F4, lighten up quite a bit at F5.6 and are even at F8.
F4 very good side to side indoors, F5.6 and F8 are great, F11 - stop here for good quality. IS is very good. Infinity seems softer wide open on sides at F4.
I hand held 16mm and got sharp photos at ľ second. (Sharp enough to see stitching clearly on a pillow on a chair)
My copy of the 16-35 F2.8 is sharper in the center but way worse at the edges; they never get as good at any aperture as the new F4 at F4! My copy of the 17-40 is slightly sharper at F11 (where I do a lot of architecture) but for hand holding and wider F stops, the new F4 IS seems better for me, and is way more even across the frame.
Here are some sample shots. The crop is 100% of the tree indoors at the edge and my other lenses canít do it this sharp at any aperture.