And here come the posts about back-focus issues in 3, 2, 1.
Many claimed 50L is soft. I like my at f1.4 to f1.6. Photo below shot with JPEG straight out from camera. Only resized to post here.
And that is justification for the 50L right there. With te 50 1.8 it's only really good around f/2.2 or above, the 50 1.4 prob around f/2 but if you actually want decent quality with nice bokeh at f/1.4 - 1.8 you're gonna need the 50L. Oh and it wont break so easily either.
I've owned two copies of the 50/1.8, and a singe copy of the 50/1.4.