I use that ancient Canon 50 F/1.4 and fight through its 'hunty' focusing because it delivers sharper images than the 50 F/1.2L (as I rarely shoot wider than F/1.8 ). From F/2.8 to F/4, it's a super sharp lens, but it has a boatload of limitations. This Sigma might address a host of them.
I wish this was smaller though. The size of the 35 1.4 is what has kept me from buying it. I want a small and inconspicuous prime. Getting a large lens merely for the f/1.4-2.8 is not worth it (STRICTLY in my case). I WILL buy a smaller Canon 50 f/1.4 if only they give it a darned ring USM!
That lens is a 2014 lens for certain. It's called Canon's new 50mm non-L. It will be an 'non-L IS refresh' like the 24/28/35 before it -- ring USM, internal focusing (no length change), better build quality, much sharper, and with IS. Probably $799 to start and walk down to $599 after the first year (all of the other non-L IS refreshes showed this behavior).
It will be much smaller than Sigma's new offering (I imagine roughly as big as its 'twin', the 35mm F/2 IS), but I would wager not quite as sharp. But keep in mind that each of the non-L refreshed were big sharpness improvements to their 1990s predecessors, but the 35 F/1.4 Art lens was even sharper than those. I expect that trend to continue.
So I'm still waiting for that new Canon 50 to stack up against this new Sigma. If it is only 90% as sharp but has IS and is as small as I think it will be, I'd take the Canon 10 times out of 10.
Yeah, but the IS is rumored to be 1.8 or even 2. Now, will it be remarkably better than my 50 f/1.8? Dunno, the 35 f/2 IS has had me thinking for the longest time, and I might just pick it up some time, but I wish they kept the f/1.4 and just made the 50mm better- sigh!