This lens is not new, or universally regarded as magnificent, but I have bought one and tested it here: Canon EF 50mm f1.2 L Test: Legend or Lemon?
Going into ownership I knew that there was some significant negative press on the internet and some issues to watch out for. I figured some of the problems people encountered were due to things like not conducting focus calibration, recognising the impact of f1.2 on achievable focus accuracy and the lack of a floating element to correct for very close focus. I therefore went in eyes wide open and have come away with the conclusion that this lens is has some very real snags, but as a 'people machine' it is truly wonderful.
My copy is nice and sharp wide open at f1.2. It has lovely bokeh, colour and contrast, but most of all it has a lovely organic and roundedness at the widest apertures that hugely appeals to me. It has some modern characteristics, but is a departure from the 'sharp everywhere always' lenses we are seeing more and more of. Personally, I love it. YMMV, but I venture that if you do not own this lens and you are an EOS shooter looking for something in this focal length that has something special about it, you will love this lens.
It is not a good 'general purpose buy'. It is quite large and fairly heavy. It is expensive and has clear weaknesses (field curvature, weak edges at wider apertures etc), but oh boy, for the human subject, IMHO, its incredible. I am so glad I did my research and went with this lens. It is exactly what I was looking for in terms of output.
I can't wait to get stuck into some serious work with this lens.