The bokeh, and the odd "sigma" color pallet apparent in the galleries that me and others have mentioned are issues to consider.
You see, if I and others can afford the top quality stuff to begin with, or at least justify paying such a high price just for the 'colors' (which, as important as color fidelity is, sounds utterly ridiculous in paying so much for in this digital age). Then there wouldn't be these competitive brands releasing such products, and frankly, doing good business.
If you've got a 35L already and are upset that people are dissing 'your' lens and don't want to feel left behind then get the Sigma for yourself, while if you're happy with the 35L then KEEP IT! Sigma is here for people like me, who can pay just enough for their new lens and not quite enough for even a used copy of Canon's 14+ year old design.
So what if there's hype, it's there for a reason, let the potential losers be disappointed and run back to Canikon and have them say "sorry, you're right, I'm never getting Sigma" and then there will be winners who produce great shots with the lens regardless.
And bokeh rendition and how good/bad it is is so subjective, if it's busy, swirly albeit still smooth and not full of CA then that's great for me (sure, busy and smooth don't always go together), but I personally like what the Sigma is producing in terms of bokeh, and after reading some blog posts from real-world pros that ditched their name brand 35mm lenses for the Sigma and report how happy they are with it (with the "yeah the colors don't pop as much, but I don't care" response), I feel fairly confident that I'll personally report back that I'll like the lens once I get it.
As for their history of poor AF, yeah, my 70-200 OS is fast and spot on, never missed, although it needed some AFMAing after I adjusted I never had to touch that, if anything my Canon 5DII that has LOUSY AF might not lock focus. Oh, but we're not talking about camera but lens AF, reason for Sigma to bring out the USB lens dock to fine adjust the lens in the future... don't know how effective that'll be but at least they're not selling-and-forgetting.