Compared here (on a Canon 40D) are the Canon 50 f1.4 and Sigma 50 f1.4 primarily. I have included the Sigma 30 f1.4 and Canon 17-55 f2.8 IS for the center and border tests, but my primary goal is to compare and comment on the two 50mm primes. If you view the actual tests, my comments are there to read in the "notes" section underneath each image.
Also, a quick SHOUT-OUT to POTN members "ef2" for letting me use his Sigma 50 and "silvex" for letting me use his Canon 50. [The 30 f1.4 and 17-55 f2.8 IS are my own copies.]
Since the Sigma 50 F1.4 EX DG HSM is the one everyone wants to hear about, let me make some general comments on it (especially vis-a-vis the Canon 50 f1.4 EF). In terms of its overall build quality and handling, it is superb. Very solid in hand, robust in feel, chunky 77mm front glass, focus ring is dampened nicely (albeit a little gritty). The Canon EF lens feels a bit inferior and "shy" when put side-by-side next to this EX big boy. But the Canon has a nice lightweightedness to it, along with a much smaller filter size (58mm). And it isn't made of junk either. Focus ring is smooth (though not like an L-lens), and overall build is good. But the Sigma in this regard is the better unit per se.
As for imaging quality, the Sigma is also the better pure optic (or the better optic per se). Right from the get-go at f1.4, the Sigma delivers slightly better resolving power but quite a bit better contrast reproduction. It also lacks the CA levels and halation-effect of the EF lens. At the borders the Sigma is also sharper and lacks the strong shading issue of the EF lens. [No doubt the 77mm front element helps here!] But I would rate the Sigma as "excellent" and the Canon, in comparison, as "good-to-very good". From about f5 and on, both are running very strong; but at the "money apertures" of f1.4-f4.0, the EX prime is better, from center to edge. As for flare performance, this is another area where the Sigma is better. I compared the lenses, back and forth, several times, with the sun shooting near or into the frame, and the Sigma resisted flare well while the EF lens had some very evident "hot spots" and reddish streaks filling the VF. And regarding bokeh, I much prefer the Sigma.
One thing I noticed after viewing many frames between the two lenses is that the Canon 50 renders a narrower field of view than the Sigma 50. This is evident even at infinity focus, so if the Canon is a true "50mm", then the Sigma is more like a 47mm to 48mm lens. Not a huge difference, but certainly one that is noticeable with both lenses shooting an identical subject and distance.
In terms of AF (UPDATED NEGATIVE COMMENTS), this may come as a surprise, but I find that the two lenses are comparable in speed, noise, and accuracy. So even though the Canon has "micro USM" and the Sigma true "ring HSM", I am finding that their AF speed is practically identical (arguably slight edge to the EX) and both pretty darn quick. The Canon's focus barrel does physically extend and protrudes out while the Sigma's recedes pretty deep "into" the lens at infinity focus. At MFD the Sigma moves "outward" though never actually protruding physically like the Canon. The Canon's AF noise is a bit more mechanical while the Sigma's is more of a "swoosh swoosh". But their noise levels are not that far apart; they are both fairly quiet. And as for AF accuracy, I find them both to be comparable as well (unfortunately in a negative sense). After a few more days of usage, I definitely get OOF shots with both, more so than I would have expected (my Sigma 30 f1.4, for example, is significantly more reliable). I find that the Canon really struggles to nail AF in harsh lighting more so than the Sigma; however, I find that the Sigma renders a good number of misfocused shots for subjects within a few feet. Strangely, it nails AF more accurately at infinity than close up. I basically notice a front-focusing issue with a good number of my images for closer subjects with the Sigma. If I use MF, it's quite impressive.
So here is my SUMMARY-ANALYSIS between the Canon 50 f1.4 and Sigma 50 f1.4 (primarily judging based on their "f1.4-f4" performance):
Resolution (center): Sigma gets the slight edge here.
Resolution (edge): Sigma is the definitive winner here.
Contrast: Sigma here too.
Shading: Sigma again.
Distortion: Comparable (nothing to worry about).
Flare: Sigma the winner. [Note: Sigma includes a lens hood.]
CA: Sigma by a good margin.
Bokeh: Sigma by a very good margin.
Build: Sigma again.
Handling: Both are good though I prefer the feel and "big-boy-ness" of the Sigma.
AF (noise): Comparable.
AF (speed): Comparable again.
MFD & Magnification: Comparable.
Price: Canon 50 f1.4 $325, Sigma 50 f1.4 $500, Canon 50 f1.2 $1400 [All B&H Pricing]
USA Warranty: Sigma 4 years, Canon 1 year.
Bang for Buck: Very tough call as the Canon is signficantly cheaper. Draw.
[Note: It might seem that the Sigma takes practically every category in my summary above, and in an absolute sense, it does. But I do not want to communicate the notion that the Canon 50 EF is a poor performing lens; far from it. Considering its price-point, it's size and weight, and its overall quality, it is still an excellent choice. Do I think the Sigma is better? Yes, I do. But it also costs almost $500 USD and is larger and heavier and requires 77mm filters. How would the Sigma fare against the Canon 50 f1.2 L? I personally don't know; you can head to The Digital Picture and view Bryan's comparison there (where the Sigma actually puts in a very solid performance). Regardless, the Canon 50 EF is a good lens at an excellent price-point. And if you look around even a little bit on various imaging forums, you'll see that the Canon 50 is capable of tremendous images when put into the right hands.]