I have owned 3 copies of this lens over the years. They have all performed consistently and in general I have been very happy with them. The only reason I don´t have it at the moment is because I use the 85mm f1.2L II and the Sigma f1.4 instead. There are in my view three main reasons to buy this lens. 1: It is in general the best available 50mm with a canon mount. 2: It has a build quality second to no other 50mm and 3: It gives you very good results at f1.2.
If your problems with this lens is that it does not outperform other lenses at f4, don´t buy it. It is supposed to be used at f1.2. If the only IQ quality you care about is sharpness, don´t buy it, because that is not where it shines. If you have problems with focus shift at MFD, don´t buy it, because it is not a macro lens (focus shift has never been a problem for me, because I always used it wide open and seldom closer than 1.5m). But if you are looking for short DOF, good colors and great bokeh and 50mm is the focal length you are looking for, this is it!
Reading some of the posts here, I feel the same way as I do reading audiophile posts (hifi and music is my other addiction), where the quality of a setup is judged by how heart stopping the punch from the big drum is. Not much music in the frequencies down there. Yes I like sharpness, but I like good color, 3D pop from shallow DOF and beautiful bokeh more. For those of you with serious problems with this lens, and I am assuming that you don´t have bad copies, should probably look for faults else where.