Recently (as in, the last few weeks) I have been reducing my Canon DSLR lens 'collection' of 8, and have whittled it down to 5. That is, I sold 3 lenses (using gumtree in Australia - highly recommend it as a useful medium to advertise and sell in one's own geographical area.)
Among those I sold was the Canon 50mm f1.8 (for your info, the other two were the Canon 28-135mm USM IS, and the Canon 100-300 USM).
The 50mm f1.8 had superb qualities in some settings, (and yes, I had a 'decent' copy - in fact I initially had a copy that back focussed, and returned for this one which was better). On my Canon 350D body, its focus performance was weak. It was better on my Canon 7D, however still not to my liking.
I have decided that I probably won't get any lens that doesn't have USM / HSM (equivalent) focussing. The 'jittery-ness' and the noise and lack of focus flexibility (eg slower speed, apparent lack of fine accuracy, full-time manual focus) bugged me with the 50mm f1.8 in particular, as I mainly wanted it for critical applications (between f1.8 and f2.
Friends, I do realise "there is a very fine depth of field at f1.8" - I am aware of that, and I have used other lenses with a similarly shallow depth of field. The 50mm f1.8 just didn't meet my needs consistently. Focus accuracy and usability were the main issues I had with mine - even after I applied a systematic use - and noticeable improvement with my 7Ds manual focus adjustment (MFA).
So at the moment, the only prime that I have is the Canon 100mm macro USM (nonL)- which is a lens I really enjoy using. Obviously it serves superbly as a macro (where I usually use it hand-held with live-view). But in addition I ended up preferring it over the 50mm f1.8 for casual portraits and subject isolation (eg objects / still life). I expect to keep this lens. My copy focusses fast, consistently, even with very small dof settings. I get a very high keeper rate, whereas the 50mm f1.8 keeper rate was so-so, which annoyed me. When the 50mm f1.8 shone, it did well. But my 100mm macro shines consistently!
My next 'dream' prime / wishlist is a fast prime 50mm lens that has great bokeh, fantastic sharpness and good contrast WIDE OPEN, and has USM focussing. F1.8 is enough for me, though of course a f1.4 would even be nicer, but not necessary. I don't want a big, expensive lens. Several reviews show that at f1.4, the Canon f1.4 isn't really sharp / contrasty. So f1.8 is sufficient. The Canon 50mm f1.2 L is really beyond what I want. Most important to me is that around f1.8 or f2, I want it to be GOOD, very good on all levels mentioned above. I don't need weather sealing for it. The Sigma 50mm f1.4 has some good reviews, but also there are QC and focus issues.
An icing on the cake would be for it to have a good (eg 4 stop) IS. A man can dream, can't he? From the sale of my 3 lenses, I have put the funds aside to save up for such a lens. Even if a 50mm f1.8 III (ie new / replacement lens) will be above what the current 50mm f1.4, I'm happy to pay for it, if the IQ is good. If there will be a 50mm f1.4 II, I hope it's similar to the size of the current Canon 50mm f1.4. I'd be a bit disappointed if the price of a Canon 50mm f1.4 II will be significantly higher (even if its not an L) than what they could make a 50mm f1.8 III for.
Time will tell....