I just purchased this lens refurbished a few weeks ago, and although a tad softer wide open than when stopped down, the image quality is still quite excellent.
Stopped down to f/2.0 the sharpness is amazing on my 5D, sharper than my 85 f/1.8 which until now has been my favourite lens, and noticably sharper than my 24-70L (which I am using less and less as I discover the joy of shooting with primes).
I don't have the 135mm f/2.0 yet, that will be my next purchase and from all reports maybe the only lens I'll ever own that could beat this. I can't afford the 50 or 85 1.2 lenses, and judging from the results I can get from the cheaper equivalents, I doubt the difference would be all that much anyway.
My 85 produces slightly better bokeh when stopped down, it's not quite circular on the 50, and the focus ring isn't quite as smooth either, but other than this I can't really pick any faults. This lens is quickly becoming a favourite, it has stayed on the camera since I bought it.
Very impressed so far!