There is some corner softness wide open, but at f/8 improves a lot and is on par with the 16-35mk.ii at that aperture.
Wide open the 16-35mk.ii has better corner resolution. For the money it is a great lens at about 50% the price of the 16-35.
Even at F4, it is pretty usable as far as resolution goes... here's a shot I took of it at F4 ... gives you an idea of using it wide open on FF, you can see the corner softness, but the image otherwise tells the story you want to tell.