I haven't used it but from the reviews I've seen it does say that sharpness does drop towards the corners when wide open, worse at 30mm, all of which can be improved dramatically by stopping down to f5.6-8. TBH this is what I'd expect from a wide angle, and is not too dissimilar to any lens really ie softer edges wide open. Now of course without trying myself I can't comment on how soft this is.
I have also read about sample variation which is disappointing considering how new these lenses are and that they are supposed to be premium lenses. It does seem that sample variation seems to be accepted amongst most manufacturers these days
All this being said, I think we're at a point now where we're generally getting picky. The sharpness of modern lenses is generally very good and a far cry from lenses of yesteryear. I've seen some mixed reviews about the 24-70mm f4, and even DXO's test scores is poor, but my copy is one of the sharpest lenses I've used, beating all the copies of the 24-70mm f2.8's I've had.