I have one of the much bashed 1.4/50 of the lower serial numbers. I can not say anything about the better ones but as far as I have read, the newer ones are only better wide open. From 2.0 on the newer ones aren't much better.
For the really shallow DOF I do not use mine wide open. In dim light, I just benefit from the slightly brighter finder than my 1.8 MIJ. Otherwise I prefer the latter for its smaller size.