Many people complain about the poor build quality and less than modern autofocus system. Mine works great on my 5D11, and it did not need dialing in with micro-adjust. People also complain about poor optical quality @ F 1.4 and F 2.0. I have compared it to alt lenses 50mm F 1.4 Takumar, 50mm F 1.4 Zeiss C/Y, 50mm F 1.7 Zeiss C/Y, and 50mm F 2.0 44-2 Helios. Of these, only the 50mm F 1.7 Zeiss C/Y noticeably outdid it in all areas using F 1.7, F 2.0 and F 2.8. Past F 2.8 it is a good as any of them, better than the Helios by far.
The Takumar appeared to be a bit sharper wide open than the Canon (edges a bit crisper), but has a warmer color rendering, and the Canon actually has more contrast than the Takumar wide open (which is a bit of a contradiction because normally the lens with more contrast will appear to be sharper) My copy of the Zeiss 1.4 to MY eyes was no better at wide apertures, my Helios is far worse @F 2.0.
Based on my experience, it has behaved ok with none of the internet ballyhooed focusing problems of the Sigmalux, or the 50mm F 1.2L lens. Considering I acquired it for free as part of a deal when I sold all my film equipment to fund my digital system, I cannot complain about it at all. When the day comes the micro usm motor fails, I will probably start using my Takumar all the time, rather than replacing it:)