The 50L comments drive me a bit batty. A few questions/observations:
1. People want f/1.0-f/1.2, but sharp to f/2.8
* The original Canon 50mm f/1.0L was much less sharp than the f/1.2L. Though the aperture is not as wide, the 50 f/1.2 was deemed overall better looking with better sharpness/contrast while retaining much of the look/bokeh quality of the 50mm f/1.0, and eons less lens flare that was distracting in the original 50 f/1.0. As the aperture gets wider, keeping the lens sharp gets harder.
* Why is there not an f/1.0 lens that is sharp for every focal length? Might it have something to do with the impossibility of doing so without other massive compromises? Why is there no f/1.0-f/1.2 24L, 35L, 135L, 200L?
* Pointing to the 85L II is irrelevant as it is a totally different focal length, needing a totally different optical formula. The 85L II also has a lot of compromises, including slow focus, extending front element, lack of weather sealing, fragile rear element, large size, focus by wire, weight, price - it is a stunning lens, but it too has its share of problems.
* Again, the original Canon 50mm f/1.0L was much less sharp than the 50mm f/1.2L and on top of it had all of the disadvantages of the 85L II (slow focusing, fragile rear lens element, large, heavy, etc).
* Other 50mm lenses f/0.95-f/1.2 like the Leica Noctilux are similarly not razor sharp.
2. People want no focus shift
* Focus shift is a symptom of spherical aberration on wide aperture lenses
* If you correct all spherical aberration, the bokeh looks less attractive (see Sigma ART f/1.4)
* Most wide aperture lenses have some amount of focus shift, including the Canon f/1.4 and the Leica Noctilux; Since the Canon 50mm f/1.2 has purposely uncorrected spherical aberration to increase bokeh quality, there will be more focus shift and less sharpness (also due to the wider aperture).
* For creamiest bokeh and wider apertures at 50mm, some focus shift is going to be necessary
* A better solution to focus shift is introduce the ability to allow the camera body to make autofocus corrections based on the attached lens, focus distance, and apertures selected. This would allow the lens to retain the creamy bokeh without focus shift.
3. People want a Sigma ART f/1.4 clone
* Sigma f/1.4 allows 50% less light than Canon f/1.2. Less light can mean higher ISOs = less contrast, less sharpness, more noise.
* f/1.4 has about 14% less subject isolation ability than the 50mm f/1.2 due to greater depth of field at the narrower aperture.
* Sigma's bokeh is not as smooth as the f/1.2's, likely due to all the corrections to get maximum sharpness. For the same reason, it has less focus shift.
* The Sigma has the size and weight of a zoom lens or prime telephoto, and is a bit unwieldy for a 50mm lens.
* The Sigma has documented focusing issues on outer points on some bodies, not just focus shift, but failure to focus accurately even wide open.
In the end, I feel asking for an f/1.0 - or even f/1.2 - 50mm lens that has all the beauty of the 50L and all the sharpness of the Sigma with the light and subject isolation capabilities of f/1.2 and less than $5000 is just too much to ask for. I would say if you would prefer a less challenging lens, the Canon 50mm f/1.4 is probably your best bet - or wait for the 50mm f/1.8 IS. If you are a a sharpness junkie, get the Sigma. But otherwise, if you want a lens that is awesome all around for portraits, you can't beat the 50L and hence I see no reason to replace it. There are a multitude of options in the "sharp as a tack" category, I see no reason to extinguish one of the few amazing portrait lenses that is out there (along with the 85L II) to satisfy sharpness junkies.