I had three 24-70's and they all had QC issues that made them unusable for serious professional work. All eBay'ed or traded in. Out of desperation I bought a pre-owned 24-105 just to see me through until the 24-70 MkII eventually gets released.
But guess what? The 24-105 is a cracker of a lens. My copy is pin sharp wide open. My main gripe is the very shallow hood. It's the same hood as the 17-40 f/4.
I shoot with 1D MkIV bodies and with the amazingly clean files I'm getting at higher ISO's, having an f/4 lens is definitely not the issue it once was. This factor will gain even more traction when the X body ships. Not only will it deliver even cleaner at high ISO's, the AF at f/4 will be more efficient than with the MkIV.
All the same, when the new 24-70 f/2.8 ships, I'll probably get one, but the 24-105 is a keeper.
Conversations with other photographers point to a general dislike of the QC of the 24-70 f/2.8, though there are indications that the later builds have overcome the QC issues. So if you do go for a 24-70 f/2.8, don't get a second hand one that may be offloaded because of QC issues or may be an early build, get a brand new one.