I have the Canon 50F1.4 and the Sigma 50F2.8 macro lens. Once you are at F2.8, any of the 50 mm lenses should be pretty sharp. The Canon 50L is not in my bag, largely because of price. I really don't see much of a need for the extra speed, beyond F1.4. However, I would be very interested in an updated 50 with real USM and IS, even if an F1.8 or F2.
The Sigma is a very nice lens, and AF is accurate, if slow. If Sigma managed to make an F1.4 version with AF that is comparable to Canon's, I'd get one.
I am waiting for an updated F1.4 from Canon.
Castleman has a website comparing the different Canon 50mm lenses, and it is worth a look. SLR Gear also has a 50mm comparison the OP might want to examine. According to Castleman, the F1.2 lens is sharper at F 1.4 and F2, with better flare resistance. At smaller apertures, the F1.4 lens was reported sharper. Clearly, the F 1.2 lens is designed for use in very low lighting, where it excels.