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.
Big +1. As soon as this lens was announced, I felt that the only way that it would be a success is if it destroyed the 24-105L optically. It didn't seem to offer any other compelling reason to purchase: focal length the same. IS/OS. Same maximum aperture.
Downsides included: Heavier and larger front element. Not weather sealed. Sigma's reputation for sometimes inconsistent AF accuracy. Third party lens with potential downsides that come with that.
I think the consensus out there is that in SOME ways the Sigma is better optically, but it certainly doesn't blow the 24-105L out of the water. That is going to make it a hard sell. Just out of curiosity: a lot of you own the 24-105L - are you planning on selling it to get the Sigma?
I don't own the 24-105L right now. I've owned two copies in the past and liked them considerably. I own let my last copy go when I got the Tamron 24-70 VC and found that I wasn't using the Canon anymore. But if I owned the 24-105L right I certainly wouldn't be selling it to get this lens.
I bought my EF 24-105L second hand. Got it fairly cheap too. I knew fine well going in that it was not going to a lens that had amazing IQ and that was fine because I just needed something to work with in the general focal range. For that it performs well, especially from 35mm onwards I'm seeing really quite sharp images. 24mm performance could be better but I can live without since I have the 17-40L and now the Samyang 14mm (on my 7D it gives an almost 24mm look). The weather sealing has came in handy when on vacation / at te beach etc. the lens isn't light by any means but a decent amount that I can handle.
Will I change for the Sigma? Nope. The Sigma's bigger and heavier and not sealed. Don't give a damn about a tiny bit extra sharpness at the wide end.
I think for those who don't need the extra range the 24-70 f/4 IS beats both the Sigma and 24-105L and provides a nice third option. If I was to change I'd prob go for that one, just wish it was a little cheaper though.