I had the Sigma 50 ART for a week or so before I sold it. It is sharp, and the bokeh is good enough, just looking at the bokeh. The main problem for me was the lack of depth rendering. It made subjects and images look flat. The EF50L is four leagues ahead in that department.I've never owned the EF 50/1.2L so I cannot comment on what it's like to use, but I like the photos it can take, even if it isn't the sharpest lens. What I don't get though, is the comments I see above criticising the Sigma 50 Art, eg the comment that the Sigma 50 Art is more like a Canon EF 50/1.4 with better build quality. The 50 Art renders a little differently from the EF 50/1.2L, and at it's best I would give the edge in bokeh to the Canon, but that edge comes at the expense of sharpness and I think the 50 Art bokeh is still right up there (for a 50mm).
I particularly don't understand comments criticising the 50 Art's bokeh but praising lenses like the RF 50/1.2L and the EF 85/1.4L IS. The 50 Art is quite similar to those recent Canon lenses, in my opinion. They are sharp but that sharpness is accompanied by a slightly different bokeh than less sharp lenses such as the EF 50/1.2L.