I would only buy if it was something I was legitimately going to use a lot.
If you feel like it's worth it to throw down 1500 on a 50mm 1.2L, and feel like you'll really use that shallow DOF that often, then go for it... if not, stick to more affordable options, or primes you'll use more.
I personally have no need for that kind of ridiculous DOF unless I'm shooting film, and in that case, I'd just rent a leica M9 with a noctilux.. which is where I say, rent if you need shit like that. Get more affordable stuff in the meantime.
For that 1500 you'd spend on the 1.2L, you could buy a 50-1.4, and several other sweet lenses...
or you could just buy 15 Plastic Fantastic 1.8's... that's how ridiculous that shit is.
Have a nice day, Bluename.