I just finished testing my copy of this lens (it arrived this afternoon). The results were shocking and disheartening. I really wanted to like this lens, I really did, but the truth is what it is, and this copy is a dog.
F2.8 through F4.5 were horrifically soft from 24mm up to about 42mm.
Since this is marketed as a 24mm F2.8, I would expect it to be crystal sharp in that range. The facts are that the 24mm had an extremely noticeable blur up to F5.6, where there was a sudden and dramatic improvement. (The lens remained impressively sharp at 24mm and F5.6 on up - but this softness is NOT acceptable for a $1200 lens! A basic 28-90 was actually sharper than this lens was.)
I am returning this 24-70 lens and will request a second one, which I will test as well.