I am in the market for a 50mm lens for my Canon DSLR. I've read plenty of reviews, and all of them have great things to say about both lenses. From a scientific perspective, the f/1.2 performs better wide open, which is to be expected given its price.
My question is, from a real-world perspective, from people who have actually used both lenses, is there any real reason to get the 50mm f/1.2? It costs over four times as much, which really isn't a "problem", per-se, but not an amount of money I want to spend unless I absolutely have to.
My primary use for this lens would be astrophotography and probably some portrait and close-up nature work. It will initially be used on an APS-C sensor, but I plan to upgrade to a 5D Mark III soon after it is released. When it comes to astrophotography, I intend to use such a lens wide open, or perhaps up to one stop down from wide open, as light gathering ability in pitch black is the main idea.
Based on Matt Grum's answer, I am also interested in hearing about competing fast 50mm lenses. I had not thought of third party lenses before, as I've always bought Canon, however if a third-party lens is optically just as good or better, for a better price, I'd be interested in hearing about them as well.