A lot of replies above highlight the exact reasons I don't currently own a 50mm.
Canon 50mm f/1.8 - poor AF (hunts and inaccurate, I had 2), poor bokeh
Canon 50mm f/1.4 - not sharp enough wide open, micro USM AF not so reliable
Canon 50mm f/1.2L - too expensive / too big and it has that focus shift issue
Sigma 50mm f/1.4 - too many reports of AF issues (also with different camera bodies)
other 50mm's (eg Ziess). I need
The Sigma 50mm would probably actually be my 'choice' if I was forced to choose. I'd go to a local bricks and mortar shop where I could try before I buy (the actual lens) - and I would use a shop that has a good return policy.
But in reality, I'm waiting for a new Canon 50mm USM, hopefully at f/1.4, but I will consider even if it's a f/2, and even if it has STM focus (rather than true USM).
IS would be an additional bonus (not quite an essential for me, but definitely a plus if IQ is still great wide open).
The new Canon 35mm f/2 IS USM is 'promising' - in that, I hope Canon keep the same format / standard in developing / producing a similar 50mm!