At first I thought Canon must have sold all their B-stock Canon 50 mm f/1.4 lenses to Yongnuo who changed the labels and lens mount to plastic. Upon side-by-side review, there seems to be more differences. For example, the front plastic round rim that moves back and forth as lens focuses seems to be different in thickness and the details in the slope leading to front glass.
One can pick up the Canon for $350 new right now, and this Yongnuo is listed for $300, which wouldn't make any sense for the buyer, EXCEPT if Yongnuo somehow improved on the Canon, i.e. more and round aperture blades for better bokeh, a little sharper wide open...somehow. Then, it gets pretty interesting IMO.