Some interesting observations from Optical Limits on 24-70 lenses in general ...
"Let's start this review with a little discussion - are 24-70mm f/2.8 lenses an attractive offering or just overhyped? Within our local team that's an open question"
http://www.opticallimits.com/canon_eos_ff/1052-sigma2470f28art
"lack the greatness that should come with these price tags"
http://www.opticallimits.com/canon_eos_ff/1052-sigma2470f28art?start=2
I had the EF 24-70 mark II and sold it as I found I rarely actually used it ... as I found my 16-35 f/4 was fantastic for landscapes (although I increasingly use panoramas these days a la Thomas Heaton) and I used primes and 70-200s for portraits ... displacing the 24-70 out of my bag almost always.
On a day trip when I want to travel light with a simple configuration (one camera, one lens, Lowepro Toploader case) I find my 24-105 f/4 mark II on either a 5Ds or 5Div competes admirably with the 24-70 Mark II off-tripod (in the f/4 - f/5.6 range) and gives me substantially better range.
True, its not a perfect lens for portraits begging the creamiest blur, but a great all-rounder and, in my opinion, solidly deserving of its L status, and at 105mm can produce some surprisingly good portraits.
So, personally, I can't say I'm too fussed whatever they do !