I would buy this lens to use at f/2.8. If it doesn't perform at f/2.8, my 24-105mm L will perform at f/4 or f/8.
I've had five of the Mark 1 version of 24-70mm L's, and was a bit disappointed in all of them, so I'm pretty cautious about taking the plunge.
I'm also interested in CA's, I tried a Nikon 24-70mm f/2.8G with my D800, and CA's at f/2.8 were horrible.