16-35 f2.8II was the first lens I ever bought when I got into photography again after 15 years. I was very excited that I had bought (without reading any reviews) a great Canon lens. It was expensive. And then reality hit hard after I took my first few pictures. I could not believe how bad the corners were. Then I slowly realized it was a bad lens and got rid of it. Attached is the photo which I took. Hated the corner sharpness.
Now I have the Zeiss 15mm and 35mm. Canon 24-70 II. But I think this new lens is very tempting.
I love the 16-35 f/2.8 II for the creative possibilities it provides but full agree with the corner image quality comment and share the same frustration - see bottom left corner in the picture below. I like doing inside architectural work where I've recently tried to use the TS-E 17mm f/4 instead of the 16-35 f/2.8 II. For this I consider the new 16-35 f/4 with a 4-stop IS superior over the 16-35 f/2.8 II - even more so if the corner quality is as good as the MTF charts suggest.