Lots of threads on this out there, but here's my take:
I have owned all three Canon 50's. 1.8 has a tiny focus ring and throwaway construction. Great to start with, but I can't rely on it to use for weddings, etc.
1.4 is much more solid, and has a good focus ring. There are autofocus motor issues, and my copy suffered from this. The AF just stopped working one day. It started working again once I jiggled the focus ring around, but it happened once more (both times during shoots), and once I looked it up and found out that many people have issues like this, I had to get rid of it. It's also not the fastest focusing lens, and the colour cast just isn't quite what I like.
I picked up the 1.2 and I absolutely love it. It feels so solid, it focuses very quickly on my 5DMKIII, and it takes wonderful photos. I like the colour cast much better than the 1.4's. It is also weather- and dust-sealed. This is important. I wouldn't risk shooting in even light rain with the 1.4, and when it comes to weddings you need to get the shots. I can't let a tiny bit of water scare me.
In terms of sharpness and overall image quality, it can sometimes be a wash. I hear lots of people say "I can't tell the difference between images from any of the 50's, especially between the 1.4 and the 1.2, so I would just buy a 1.4 and save a bunch of of money."
Then when I ask those same people about reliability, they say "Well, I have had to get my 1.4 fixed twice, but it's still cheaper than buying a 1.2." To that I say: sure, it is cheaper if you are only talking about the price paid. But what about your time? The time it takes to take the lens in or ship it away for repair. And what about the time without your lens? What about the missed images due to AF issues?
If you are only doing photography for yourself and not getting paid to do weddings, events, portraits, etc. then the 1.4 makes a lot of sense. But when someone is paying me to capture their moments, I need something a little more reliable, and something that is weather/dust-sealed.
Lastly, there is a huge difference between the 1.4 and the 1.2 The 1.2 really makes the subject pop in a "3D" kind of way. It has its own character. Will you notice it in in every shot? No, but if the photographer uses the 1.2 to their advantage, there are some images that you just couldn't get with the 1.4 or 1.8. Also, it's f/1.2 - you get a little more light, and that just might help you get a shot you might not otherwise get.