Last month I purchased the Sigma 50mm F1.4 EX DG HSM instead of getting the Canon 50mm 1.2 saving myself a bunch of money ($460 for the Sigma compared to $1,380 for the Canon) and potential headache. Click HERE to read more on the headache factor. Having used this lens for most of April I can honestly say I love it. The speed is good and the bokeh factor is so much better than that of the Canon 1.4. The only drawback was having fun with the little 58mm barrel size of the Canon 50mm 1.4 compared to the larger 77mm barrel size of the Sigma 50mm 1.4 when doing reverse lens mount macro shots. The 77mm is a bit too large to cover over the entire camera body opening and so you cannot get as clear and crisp an image as the the Canon (more light flair.) I am sure the same would be true for the Canon 50mm 1.2 with a 72mm barrel size, however I have not tried it in the reverse position.
Most comparisons you will read are between the Canon 50mm 1.4 and the Sigma 50mm 1.4 however, I feel that other than the aperture size, the Sigma 50mm 1.4 compares more to the Canon 50mm 1.2 due to size, mechanics, blade construction, reduced aberrations, focus methods, the lens coating methods, and the fact that it is weather sealed. All these important factors make the Sigma 50mm 1.4 a much better choice over the Canon 1.4 which is $400. Honestly, why would you NOT buy the Sigma for an extra $60.
I tend to buy all my lenses through Amazon because they have the best return policy and usually the best price. Make sure you test any lens you buy within the first 30 days to make sure you do not have any front or back focusing issues. (click the above headache factor link to learn more about focus problems with lenses.)
Below are a few links to get you to the appropriate Sigma Lens for both Nikon and Canon shooters. I have also included the Canon 1.4 should you REALLY want to have the ability to shoot macro with it held inverted.
CANON 50mm 1.4
Canon EF 50mm f1.4 USM Standard & Medium Telephoto Lens for Canon SLR Cameras
SIGMA 50mm 1.4 for Canon Cameras
Sigma 50mm f/1.4 EX DG HSM Lens for Canon Digital SLR Cameras
NIKON 50mm 1.4
Nikon 50mm f/1.4D AF Nikkor Lens for Nikon Digital SLR Cameras
SIGMA 50mm 1.4 for Nikon Cameras
Sigma 50mm f/1.4 EX DG HSM Lens for Nikon Digital SLR Cameras