I purchased a new 50L a few weeks ago and couple colleagues and I tested my 50L against a Sigma 50mm f1.4, Canon 50mm f1.4, and Canon 24-70L at 50mm. All test were done using 5D MKII’s shooting RAW photos (similar settings/firmware…). I must be very lucky because my 50L is super sharp and the bokeh is amazing. After comparing the photos in DPP, I prefer the Canon 50L but the Sigma (required a trip to Sigma to resolve focusing issues) was very close. The Canon 50mm f1.4 was sharp but the boken was NOT good. The 50L photos were sharper and boken smoother than the 24-70L at 50mm.
My opinion from our test (x - bad, xxxx - good):
.....................Sharp........Boken.......Build Quality
Canon 50L.......XXXX........XXXXX.......XXXXX
Sigma 50.........XXXX........XXXX........XXX
Canon 50 1.4...XXXX........X..............X
Canon 24–70L..XXX..........XXX..........XXXXX
Under certain circumstances you'd be hard pressed to see the difference between a Canon 50/1.2 and 1.4 it is just a matter of the right background and lighting conditions (and no I don't mean a completely neutral background).
What I am trying to say, and of course everyone is entitled to his opinion, that the difference in out of focus rendering isn't as absolute as a 5 vs 1 rating. Note that I am not claiming that the 1.4 is better.
Do the right thing with a 1,4 and people go "aha" do the wrong thing with the 1.2 and people will go "aaargh"...
OTOH perhaps I should curb my tongue a little, because I am far from being an expert and it is high time I let this subject rest and leave the word to others.