I had a Sigma 50 f/1.4 (wich in my opinion is a way better lens than the canon 1.4 in terms of sharpness and bokeh, IF you get a good one! And I had a good one
But the thing is, that it was a Sigma, so no Canon L build quality, no CPS, no weather sealing.
The Canon 1.4 feels reeeeeeeally plasticy, but its a really good lens for the money and really not that heavy.
The reasons why I went for the 1.2 are:
1.) It has a red ring (and in the pro world that means a lot, because other pros just don't take you seriously if you run around with a 1Ds without an L lens, its stupid but its reality....)
2.) It has 1.2 (wich is just 1/3 of a stop more (canon only calculates in 1/3rd stops not in half stops, that is why there is a 50 1.8 and no 50 1.7, so the 1.2 is just 1/3 stop faster not half a stop!))
3.) It has weather sealing! And THAT was the most important point for me. If I have to shoot at weddings or other stuff in bad weather (which happens a lot) I need 2 things: A fast lens, because with bad weather there comes bad lightin aaaaaaand I need a weather sealed lens. I also got the 35 1.4 and the 85 1.2 II but there are both not weather sealed!
4.) BOOOOOOKEH! The 50 1.2 has the best bokeh i've ever seen on a 50mm prime lens for canon! Thas a fact!
The big issue with the lens is the sharpness: Its just not as sharp as other L primes! Thats a fact! But what do you expect at 1.2?? And it costs around 4-5 times more thant the 1.4...
If money is not an issue and you don't need to buy more important things than a new prime, then go for the 1.2, if you need a fast, sharp 50 for less, then I'd consider the Sigma since it is a reaaaally good lens for that price!
Hope this will help you a little