My route to this lens was via a 50mm 1.8 MkII. I won't go into the various details of build quality, feature difference as there are many reviews that address these areas.
My 50 1.8 while a nice lens had slight front focus and wasn't nearly as consistent shot to shot with regards to focus accuarcy at most aperatures under f5.6. I was left with either sending it into Canon for calibration or getting the 50 1.4. Many people may question how the 50 1.4 can be worth the extra money for an minimal extra light gathering ability, a slightly better build quality (certainly not L) and better bokeh. What I am finding with this lens is that the focusing is much more accurate than the 1.8. I'm finding that 95% of my pics are dead on with this lens while the 1.8 I would say was at about 70% (I'm being generous). I am also finding that the focusing under poor lighting is slower than the 1.8 which most would say is strange since it's USM. I think this is due to the fact that it is being more accurate. I know when this lens focus locks the focus is accurate and my percentage of "keepers" has gone up considerably. To me that is worth much more than the $200+ price difference between the 1.4 and the 1.8.