Upload & Sell: Off
Wow. So much to say.
At first, after only viewing the Sigma vs Zeiss, I was going to say that the Zeiss is clearly better through at least f4 (that's as far as I looked). I was going to say that I was frankly disappointed. But I decided to look at the other comparisons before I posted.
Holy crap, it simply puts the 50L to shame, by a large margin. If you leave the Sigma at f/1.4, it still remains sharper than the 50L until about f/5.6, which is astounding.
Comparing the 50L to the 50 1.4 was interesting too. The 1.4 seems to have better sharpness in the corners than the 50L. The 1.4 does seem to exhibit more chromatic aberrations however. Overall though, I would find it hard to justify the L when the 1.4 is pretty close.
Having said all that, many of these differences are much less apparent in everyday shooting.