There has been a lot of commotion about this lens. I use it frequently for weddings and portraits and its absolutely wonderful. Sharpness, color fidelity, contrast, and bokeh are absolutely top notch. Its right up there with the best and every bit deserving of the "L" moniker.
Where this lens excels is at very large apertures. If you are stopping down between F2.8 to F4, a couple things will happen: you are getting practically no benefit over the 50/1.4, and you will probably experience a slight focus shift in the image when using the single focus point. If you are buying this lens for anything other than shooting between F1.2 and F2.0, then dont even bother. Save your money and get the F1.4 version.
I can absolutely verify that this lens' focusing has been *spot on* at any subject distance when using the outside focus points. The center AF point has a slight AF shift when stopped down. A few squeaky wheels make a mountain out of this molehill, but to each their own.
For a large aperture, high quality, professional 50mm, this lens is absolutely unrivaled. No Canon 50mm has ever been better. The shots I get out of this lens on the 5D are jaw dropping. Its "wow" factor is right up there with the 135/2 L and 300/2.8 L. Seriously.
One peculiar thing I have noticed about this lens is that optically it isnt quite as good near minimum focusing distance as it is farther away. Given a 5ft+ subject distance, this lens is very sharp at any aperture. Get closer than that and it gets a little soft wide open. Stopping down a touch (probably a good idea for depth of field) sharpens things up.
In short, this is a very difficult lens, but it is truly excellent at what it was meant to do.