This is my first FM review - just wanted to give share my experiences with this lens.
Bought this lens to replace my 1,4 after having read more or less all available reviews online.
Obviously many people have experienced massive focusing issues, but I decided to risk it and hope for a good copy.
But the lens quickly showed serious issues on my 1Dsmk2. It brought it on a couple of jobs but quickly experienced all of the described focus issues. On my 1Dsmk2 it is simply unusable.
On my old 1Ds it performed better but still with a 'failure focusing rate' of above 20-30%.
I've spend a substantial amount of time driving back and forth to the local Canon repair shop. They have tried to adjust my camera, the lens and to calibrate the two to each other. Without any success.
So how can I possibly have rated this lens a 10 you may ask? The answer is Canon 5Dmk2.
These two pieces of gear appear to be the best friends in the world.
I've not encountered one episode of focus failure with this set (at least not caused by the gear - the shallow depth of field is obviously challenging to work with).
The lens have gone from being problem child #1 in my bag to being my preferred portrait lens. I've hardly used my 85 1,2L since I bought the 50mil - it's that good! I enjoy the shorter working distance to the subjects + the ability to use the lens for other things than portraits. The 85 1,2L is IMO a far more specialised lens.
To potential buyers: Do not buy this lens if you do not have a chance to test it thoroughly on your camera. You may end up with an expensive hunk of glass providing you with a constant source of remorse and grief.
On the other hand, if the two get along - you will have yourself one of the best portrait lenses available + a lens which is very versatile and usable for other things than portraits (at least on FF).