In the last 12 to 24 months we've seen an array of new lenses from Tamron and Sigma that seem to be laying down the gauntlet to Canon/Nikon in the area of providing good IQ that perhaps started with the 50/1.4 from Sigma. Why the Sigma 50/1.4? Because it tests better than both the Canon and Nikon equivalents albeit, it is more expensive however when Canon replaces the 50/1.4 with a 50/1.4 IS, the Sigma will be cheaper.
I was reading up on the Sigma 50/1.4 recently. The review on The Digital Picture (TDP) and reviews by some customers on B&H were somewhat disappointing. I'm referring to the comments about autofocus consistency (and the Canon 50/1.4 is no champ in this either).
TDP says, "Inconsistent focus accuracy is definitely the downside of this lens. It is possible that another copy of this lens would focus more consistently, but my guess (helped by feedback from others) is that my lens is representative of this model. Thus, unless you are primarily using manual focus or shooting at narrow apertures (f/4), I suggest buying the Canon EF 50mm f/1.4 USM Lens instead."
The ISO12233 chart crops on TDP suggest that the Sigma is a bit sharper, but the Imatest results in LensRentals.com's "Great 50mm Shootout" suggest that the Canon 50/1.4 is a bit sharper. Imatest results on Photozone.de also suggest that the Canon is mostly sharper.
The first review that really showed up the difference between the Canon 50/1.4 and the Sigma 50/1.4 was on dpreview. For example, use this URL:
So far as I know, nobody else has presented such a complete result set as dpreview on this matter.