If I was in the market for a 50mm lens I would get the Sigma F1.4 EX DG HSM. It leaves the Canon F1.4 for dead and is far cheaper than the overhyped overpriced Canon F1.2L.
See here
http://www.dpreview.com/lensreviews/sigma_50_1p4_c16/
I have heard that auto focus is a problem wide open. It is not accurate up close on some Canon bodies. It depends what you want a 50mm lens for.
Bert