I wonder if this lens is really aimed at Nikon users.
As others have said, for Canon users this may be a nice lens, but it's not an "must have." The street price of the 24-105 "L" is less. Just about every full frame Canon user who would want this lens already has the Canon version. There are currently more than 500 in stock on the Canon refurbished store (selling for more than the street price of a new "white box" version).
Basically, the market is flooded already, so I wonder if Sigma may have made a mistake with this lens unless they are just targeting Nikon users.
I admire what Sigma has been doing lately and I appreciate the competition from third party manufacturers, but I'm scratching my head over this one. It runs contrary to most other recent releases from Sigma, Tamron and Tokina – where they have either been focusing on giving consumers choices that Canon and Nikon don't offer or they've been undercutting Canon and Nikon on price with staples like the 70-200 f2.8.