I did a wedding last weekend and almost all day I used the 1dx along with 24-70 I. I'm pretty happy with the results for one that always uses fixed lenses .... Now my question is: it is worth the new 24-70, because there is a big money difference between two.
this are the images: http://vasilebarbu.com/?p=21-- hide signature --
I Love my 24-70 I and have no plans to upgrade before it breaks. Let your results speak for itself.
(you often hear people say it is soft wide open, mine really isn't. An in my humble opionion has nicer bokeh than the new one. Colour and contrast I really can't judge since I haven't shot with the lens - some say the older is better here, others the new one, but I am totally satisfied with what I see in my lens. The new one is indeed sharper, maybe except at 70mm, but in people's photography as opposed to landscape photography, I really can't see that much of a benefit for an immediate upgrade)