I have been using this lens for about a month. Here is what I like about it.1.  The USM gives me better auto-focus performance.  Worth the price diff between the f1.8?  ... I'm guessing not, but on the other hand, candids of two active boys might be helped by a speedier auto-focus.2. The f/1.4 gives me a noticable margin for low light situations3. The f/1.4 version is slightly better built and does produce better bokeh than the cheaper f/1.8 models.Here is what I dont like1. I have noticed that the lens is very soft on the wide end between 1.4 and 1.8. There is some kind of "flaring" around the bright spots.2. I have taken identical photos with my L17-40 f4 and dont see this. Although the 50/1.4 is a lot faster than my L lens.. I am sending this lens back either to Canon for calibration or to Amazon..3. The ring of the 50/1.4 appears to be loose. I dont know if this is supposed to be this way.. however for an expensive lens Canon should construct this lens like a tank. but no.. it appears to be cheap.4. The 50mm/1.8 is only about $70 so it's hard to beat. I find the picture quality is around the the same as my 50/1.4. 

Question: How do the lenses work?
Answer: Here is what I like about it.1