I don't think anyone that has used all or some of the 50mm, can't really dispute the fact that the overall image quality differences from all three lenses is miniscule. I own all three, with multiple copies of the L and 1.4, and use them quite often. With the exception of the differences in stops, the major difference is build quality. I've seen the 1.8 II roll off a bench hit the ground, maybe a drop of 16-18", and explode. Not literally
, figuratively. Or have had one hit the door way and fall apart. Not much to do afterwards but pick up the pieces and bin them. That's what a 100$ lens is good for. The 1.4 and it's damn weak sauce AF mechanism... has left me stranded twice. You get about a year or so with general use before that gremlin raises its head. With the 50L, it is hard to find much difference between it and the other two in respect to IQ (at least to me). A trained eye can see the difference in bokeh but to most, not so much. It is sharper below 2.0 but I find little to complain about in sharpness from all three flavors. What I have the 50L for is weather sealing, tank-like build, low light portrait monster, great FOV and my copy seems to be pretty damn sharp. Knock on wood, they have taken a beating but survive beautifully.
The biggest change I have seen with my 50's though is AF speed... and that can only be attributed to the 5D3 and 1DX. They behave like totally different animals now. My favorite combo is the 5D3 (silent) w/ 50L.... ummm, I hear the street calling now!
Maybe the new show stopper for cheap, low-profile, sharp, etc... is the shorty forty. Still getting use to it but it is always impressively sharp on the wide end.