I have owned and used the Canon 1,4 extensively and I now have the 1,2 L, and the main difference is that the AF is 100 times better on the 1,2, color contrast sharpness between 1,2 and 2,0 is superior. However, it does have a rep for having shift-focus, which would throw many off it (that and the price). BUT you don't buy a 1,2 lens to use at f4, and shift-focus is only an issue between 2,0 and 4. The other thing is that it's quite a bit less sharp at mfd than more normal distances. I have the 35 L and 85 L also, so I can use which one is best for what type of photo. The 50 L has, without ANY doubt, the superbly best AF of the three. And what good does great IQ have to say, when your low-light image is oof?
If you want the best handling lens from canon, the only of three mentioned that is weather sealed, the best AF on a 50 and will use it around normal distances at 1,2-f1,8 the 50 L will not dissapoint you, but it isn't perfect for everybody.
The 50mm f1,4 have had some af issues, and I did with mine, and my gf with hers. I have heard good things about the 30 f1,4 and 50 1m4 from Sigma, make sure you get a good copy!
All this being said, I really like the 50 1,4 from Canon, but you can forget mf at these apertures, if not on a tripod and shooting a chessboard
You need a camera that can focus in low-light, because the 1,8 and 1,4 from Canon doesn't impress in that way.
If I had 800 usd to spend on lenses, I would get a used 50 f1,4 and a used 85, 1,8...