I've recently bought the 50mm f/1.2 and yes, it's not a forgiving lens.
I first really discovered my love of photography on Canon (especially portrait photography) with the 50mm f/1.8 II - a lens that I still own because it's still a great lens especially when thrown on an adaptor on the EOS M3.
Just before the birth of my son I invested in the Canon 50mm f/1.4 thinking that it was likely to be a good companion to my much loved 85mm f/1.8 - but after a while I realised that I didn't really find anything that I loved about this lens. Yes, it was better than the 50mm 1.8 in most cases, but there was nothing that made me really want to use it. It laid mostly unused in my lens bag until I sold it recently.
Earlier this year I decided to treat myself to the lens I'd always wanted - the 85mm f/1.2L II - and of course was blown away with how amazing it was. I then started to wonder about the 50mm f/1.2L - I knew from everything I read that this was not something to compare with the 85 1.2L, this was a lens with a very mixed reputation. And then one came up on offer on Facebook at a price I could not refuse (a very recently made one too with the silvery box and the center-pinch cap) so I took the plunge.
I do not regret this for one moment. The 50mm f/1.2 can be frustrating compared to the 85 1.2, but it does have a unique character to it that the 1.4 doesn't. Looking at some of the photos already posted here you can see it.
I am still far from mastering it, but it is nowhere near as bad as some people have made out.
If they can do a 50mm f/1.2L II USM that's as good as the 85mm f/1.2L then Canon will of course have a winner. but until then, there's a lot of life left in this lens.