A while ago I used the 50mm f1.2 lens. It is an extremely well built weighty lens, that definitely deserves to be in the L lens range. Having said that I would put it at both the low end and the high end of the L lens range.
Firstly this lens is incredibly well built and extremely sturdy, but being a lens probably would break if dropped. It also has incredible sharpness when stopped down and produces great colour with very little desaturation. also despite what has been said about its autofocus speed I think it is pretty fast considering the weight of the glass elements that it has to move.
Shooting round London was a great test for this lens as we shot in Borough market, which was quite dark so really needed the f1.2 to get a nice exposure. we also shot in Hyde park and Tate modern balcony which was quite good for landscapes and along the thames which was more street photography/ portrait style.
I tend to not like 50mm as a focal length that much, so it may have allowed me to pick up on its issues more than an average user, I just find 50mm isn't quite telephoto enough to shoot and get lens compression and not wide enough to get nice epic vistas.
My main issue with this lens is its insane price coming in at £1000-1200 it seems like they upped the price on a lens that should be more £750 just because they knew people would want the F1.2. Although this isn't really a great negative, as it would be worth it if all you shot with was this lens, rather than having it as your 4th/5th choice like me.
My next issue is the unbelievable colour fringing as you can see in the cropped image on the right, even in the non cropped version you can see the crazy green and purple of colour fringing. This is by far the worst colour fringing I have seen from a L series lens, and is one of the reasons why I consider the price high, and believe it is at the low end of the L lens range.