Just like the big brother 24-70 II, the new 16-35 f4 IS MTF charts look awesome.
My 16-35 II & 50L are on ebay.
I'll hold off until I see some actual reviews of the 16-35 F4 but I suspect I'll be joining you in putting my 16-35 II on ebay as well... (Especially since I take alot of video with that lens too)
Why trade in the 16-35 f/2.8 for the f/4.0? Wouldn't you guys prefer the faster aperture?
On one side, you have:
F/2.8 for 16-35 small DOF work (not a super common need)
F/2.8 for low light with moving subjects (events) or for faster shutter speed for sports
On the other side, you have:
sharper lens (according to MTF charts)
IS for low light with static subjects (3-4 stops of IS crushes 1 stop of lens speed in this regard)
IS for videography
So the short answer is -- sports/event folks likely will always need the F/2.8, but so many other types of shooters just want a sharper lens and the IS is highly desired by about 50-60% of the CR forum I'd say (there are still some unconvincables that insist IS with wide angle is not needed).
But at what appears to be a very fair price, if the MTF charts pan out in real-world testing
(and historically Canon is above board in this regard), this new lens will sell will like hotcakes.