The controversy, lies in its design. For whatever reason, Canon decided that this lens did not need to have a floating element to correct focus shift that can happen at minimum focus distance at smaller apertures. I am sure Canon has its reasons, but for most people who fork out 1.5G ($CND) buy this lens, it's inexcusable. But, is it as bad as what everyone says?
I have owned this lens for about two years. It's a lens that I frequently grab and mount on my camera when I know we are going out, and spend time indoors. I have used it on three different Canon bodies: 1Ds, 1D Mark II, and the 30D and the lens works great on all of these bodies. Does the focus shift happen? In a word, yes. Does it affect the way I take pictures? Yes, to a small degree.
The focus shift is at its worse when you shoot at absolute minimum focus distance at smaller apertures like f5.6 or smaller. Everything would be focus behind your intended focus point. The smaller the aperture, the more severe the focus shift. So, for those who take pictures this way, this is the wrong lens for them. They really should be using the 50mm f1.4 or the 50mm f1.8 lens, which does not exhibit focus shift behaviour. So, what's my take?
I am annoyed by it, but it does not stop me from enjoying this great and amazing lens. Why? I don't buy an f1.2 lens to take pictures at f5.6 or f8 at minimum focus distance. The intended use of this lens is low light at large apertures. If you shoot at f2.8 or smaller aperture frequently, you are wasting money by buying this lens. The very respectable 50mm f1.4 is great for this purpose, although wide open, it's not as sharp as the 50/1.2L at wide open. I bought this lens so that I can use it at f1.2, f1.4, f1.6, f1.8 and even f2.0 for the needed depth of field.
There are other aspects of this lens that make it stand out from other cheaper lenses. The contrast, resolving power, colour, and build quality are all excellent desirable to have in a lens.
A lens takes time to learn its quirks, strengths, and character. I am happy to own and use this lens. It's not perfect, but what is?
click on the picture for a larger version
Ryan -- 30D & EF 50mm f1.2L @ f1.2
Dillon -- 1D II & EF 50mm f1.2L @ f1.6
Marigold Garden -- 1Ds & EF 50mm f1.2L @f4