I'd like the chime in with a neutral stance. I owned the 50L for over a year and also own the nifty fifty and the 50 f/1.4. Love all 3 of them. The 50L was brilliant from f/1.2 to f/2.8. Absolutely brilliant. After f/2.8 though, it was the worst of the 3 lenses. In fact, at f/5.6, the 1.4 lens looked much better and was much sharper and if I were shooting stopped down I always went for the 1.4. Now of course I go for the 24-70 f/2.8L II because I don't shoot wider than f/2.8 anymore.
Right...I was looking for a good 50mm lens, a good all-around 50mm lens. After an experience with Tamron I have decided to now stick to first-party (Canon) lenses, at least with my DSLRs. I'm still looking for that 50mm lens and haven't really found it yet.
I typically go for the best I can afford, and in this case the 50L was in my budget. But I tried it at the store and I just could not get a good shot with it. I had no problems with the 85L so I knew it wasn't the 1.2 that was holding me back, but the lens itself. Very frustrating lens to use...especially between 2.8 and 5.6 where it seemed to back-focus considerably. Then I tried the 1.4...much cheaper, decent where the 50 1.2 struggled...but mediocre below f2 (and I lamented its lack of sealing and fragile handling). Still wasn't convinced.
But after reading this discussion it kinda makes sense now. I consider myself to be more of the "technical" type of photographer - I like my shots sharp and well-exposed, at least that's what I go for...so I tend to pick lenses that make attaining that easier for me (the sharpness part, that is). If the 50 1.2 is indeed designed for more of an artistic flair (and I have seen some great shots from it), at the cost of some sharpness, that's probably why I hated it to begin with. The shots out of it were just a bit too soft for my liking, that's all. So, hopefully there'll be a 50mm 1.8 (or 1.4) IS on the way!
I compare that to the 85 which is sharp even at 1.2 (wow!) Plus its images have "that look" to boot...