I think this lens, along with the 24-70IIL and 70-200LIS II will out perform 99% of the photographer's using it
Looking over the reviews and posted results here's my take:
The new 16-35 f4 LIS has less distortions in the far corner (less image stretching). It looks slightly wider than the 16-35IIL too. The wide open sharpness is amazing, as are the corner sharpness...stopped down the difference is less noticable. The colours and contrast look a lot more vibrant, but could be due to a 1/3 under exposure with the f4....time will tell. The 16 point sun stars are new to Canon and look different, not sure if I like or dislike....just different. It's now easier to tell which lens was used with a sunny landscape picture.
The IS unit for me is irrelevant. The time it would take for the image to stabilise would make this feature less useful to me. I miss the extra stop of the f2.8. The flare control looks very good and the new coatings look better....especially for cleaning. It's quite big for what it is....but hey, it works great. At last a wide zoom lens hood which doesn't look rediculous. This one might actually protect the lens from damage.
If I had a 24-70IIL and this lens in my bag, I might get confused which one is which...they look really simular.
77mm threads, great...but erm the 24-70IIL has gone from 77 to 82mm....step ups gonna be needed.
But the real IQ advantage which no one else seems to have noticed is the lack of CA compared to the f2.8 II L.
This new f4 lens seems to have pretty much zero CA, where as the 16-35IIL really needs correcting for most images (I have a preset just for this lens in Light Room to correct CA, vignetting and distortion). CA is a blight, correctable but an irratation for sure. I've been using my TS-e 17mm a lot more recently because it needs less post prod (ironically) than my 16-35IIL.
Looking at the results from this lens, pushes me to sreiously consider one....but i think I'll wait a year for the price to drop to more realistic levels. All you fan boys with deep pockets....go for it! It's a stunner with some clear advantages over the existing models.