I bought this as a mint condition 2012 model and have used it for several weeks. I have been using it with my 5d Mark iii. I am using it mainly to take photos of my children wide open. The lens is sharp enough in the centre at f1.2. There is a little vignetting at f1.2, which is not a problem when taking portraits. Contrast increases when stopped down a little.
I was concerned when reading about problems with back focusing. I do have some focusing errors, but believe this is down to user error and subject movement rather than a problem with the lens itself. I have not found a problem with back focusing at close distances when using between f1.2 and f2. When I have stopped the lens down to f5.6, the lens is very sharp across the frame. The colours are very attractive.
I was not sure whether to get this or the 35 1.4l but I am now very pleased with my choice. I would thoroughly recommend this lens as a portrait lens or for general use.
There are some excellent portraits on the flickr canon 50 1.2l site by a photographer taking a series of photos of people that work in Soho (whose name I forget) which shows what can be achieved by this lens in the hands of a skilled photographer.