I hope this Forum will get lots of entries!
I have the 50mm and the 85 mm f/1.2L USM. I love the 50mm perspective. I use it on my 5D. And I'm very sad to say the 85 mm is by far the more reliable lens. Not only in low light conditions. My copy of the 50mm seems to have some back focus issues. If the AF works it produces stunning pictures. Its great that you can get much closer to the subject than with the 85mm. But I always have to take 5 or even 10 pictures to get one sharp at the focus point I of my choice... The Bokeh at 1.2 is wonderful. The AF problem exists @ 1.2 as well as @f16. Except for the AF issue it would be a great lens. But AF is the most important thing when shooting people on stage, acting, performing, musicians, kids @ play etc - when you don't have a 2nd chance to "compose" the frame. In a studio you could use MF, but even with the 5D MF is a very tricky business and extremely time consuming.
I always take it with me, I often use it, but out 10 shots @f/1.2 only 50% or even less are sharp. With the 85mm 90% are sharp.