I'll hold off until more f8 results come out. My rationale being that I use the 16-35II 2.8 in two ways - low light people photography in tight settings (need 2.8, tack-sharpness less important since motion blur will be the image limiting effect) and landscape (shot at f8-11 where it appears the 16-35II 2.8 is not far behind the new 16-35IS 4.0). IS on a wide angle is not a big deal to me since I have steady hands and can shoot at 1/10 sec at 16-35 mm with a decent keeper rate, and if I'm shooting landscape, I mostly use a tripod anyway.
All this said, the new 24-70II 2.8 is so sharp that I now debate whether to carry my 24TS II every time I shoot landscape. If I'm not planning on TS movements, the 24-70II seems just as sharp and obviously quicker/more flexible. Maybe the 16-35IS 4.0 is so much better optically than the 2.8 that I will get it just for landscape. f 8.0 will tell (looks similar on the IQ tests from the digital picture site). Make sense?