I am not sure how much copy variation there is with this lens, but both of mine have been similar sharpness, although the first had the decentering issue (right side soft always) which is why I sent it back. I first tried to let Canon correct it but they were unsuccessful so it went back to the retailer for an exchange.
My current copy is stunning though. I just love the lack of CA, sharpness to the edges of the frame (corners even) and the fact it remains so sharp from 24mm to 70mm. I hear some not impressed with this lens and it blows me away... There must be some duds out there, like any lens I suppose.
I rarely used my 24-105 because it just fell flat in a few ways. Extremely versatile, but "boring". The 24-70 II changed that due to how sharp and beautiful the images are wide open, end to end. I just can't say good enough things about it honestly. I think it is worth the $2000 it is going for right now because it really is just as good as the 70-200 II, on the wider end of course. And IS? Don't miss it at all... If eliminating IS allowed them to get this level of sharpness, I am all for that decision.