Sometimes it's nice to get a new perspective on an older lens. The EF 50mm f/1.2L might be one of the most polarizing lenses in the Canon lineup, some can't live without it, while others wish it never existed. The contrasting opinions of the lens are usually because of the focus shift that some photographers experience with it. Certain shooting styles make the issue more prevalent for some.
Friend of the site Dustin Abbott has completed his thorough review of one of my favourite lenses in the Canon lineup. I love the EF 50mm f/1.2L as well as the EF 50mm f/1.0L. I think I just have a fetish for fast 50's!
Dustin has this to say about the 50L when compared to the competition:
The biggest threat to the Canon 50L has been the release of the Sigma 50mm f/1.4 ART lens. TheZeiss Otus 55 f/1.4 (review coming next week) remains the class leader since its introduction, but its dizzying price (nearly $4000) and lack of AF means that it will stay off most photographer’s radar. The Sigma, however, has image quality that charts much better than the 50L’s along with autofocus, although it can struggle with focus accuracy and consistency. It also retails for about a third less than the 50L (which retails for $1450 when not on sale). You can read my review of the Sigma here. Is it the better lens? It is certainly sharper and has better contrast, but I’m not convinced that it has as “magical” a drawing/look as the 50L.
I agree with most of what Dustin says in his review and I'd recommend the EF 50mm f/1.2L to anyone.