My first test shots left me slightly horrified by the amount of CA the 35L produced at f/1.4. On close up shots, areas of high contrast on metallic objects showed purple fringing, and out of focus areas showed a green/magenta cast. I was not impressed.
But after a few more days of testing though, the sheer quality and sharpness of this lens won me over. I checked my copy against that of other owners, and saw that my copy was similar in performance to theirs.
Away from lens torture tests and used in regular shooting situations, the 35L is capable of producing amazing results. On a full frame 5D2 it creates stunning shots, where the ultra shallow DOF and razor sharpness really does create a 3D quality. I love it, especially for B&W environmental portraits.
I don't think this lens is for everyone, it's quite a specialist piece of equipment. I wasn't sure it was for me at first, but the more I use it and get to know it, the more I like it. I'd recommend renting it before buying it.