Added to my review of the Sigma 35mm f/1.4 DG HSM is a 3rd field comparison of the Sigma 35/1.4 versus the Canon 35/1.4L.
Large images and generous crops from ƒ/1.4 - ƒ/16— full series. This is a “deep 3D” comparison affording a good look at correction for color errors, and bokeh.
Why so many comparisons? My working theory is that the Sigma 35/1.4 DG HSM is a superior lens to the Canon 35/1.4L in every situation. So far that theory holds. Were it not for my particular situation (this site), I’d be thinking hard about disposing of the Canon lens (and I probably will at some point). That said, the Sigma 35/1.4 DG HSM has yet to prove itself as a long term reliable lens, so I don’t advise pro shooters to be hasty in switching. Ditto for Nikon users and the Nikon AF-S 35/1.4G.
Rumor has it that Canon will introduce a 35mm f/1.4L II in Spring 2013. This makes sense, because the performance of the existing 35mm f/1.4L clearly shows its limits on the 5D Mark III, and a future 40+ megapixel Canon will make the 35/1.4L a non-starter.