The 50mm f/1.2 L is, along with the 85mm f/1.2 L, currently the fastest lens in Canon's line-up.  It's also a somewhat controversial lens which is the subject of considerable discussion on the Internet, some of it well-informed.  The compromises involved in the design appear to give it a 'backfocus' problem if you're using it around f/2 to f/4, with a subject distance of around 1m.  But of course, most real-world shots don't fit those parameters, and in any case if you wanted to shoot at f/4 then you'd probably be just as well off using the much cheaper 50mm f/1.4 or 50mm f/1.8.

What the f/1.2 version does give you is unmatched sharpness at apertures faster than f/2, for when the light is very low; "L" build quality including weather sealing; very pleasing background blur; and great overall image quality.  It's not the right tool for all jobs, but in certain circumstances there's nothing to match it.