Hi James.....I've never used the 50 f/1.2 (I own the 50 f/1.4). But, the Sigma 50 f/1.4 Art is being reported as sharper than the 50 f/1.2.

Here is Bryan's comparison at f/1.4. Then Sean and I have both purchased the 50A. Here is the thread. While there has been some concern with AF, thus far mine is fine. And I put it through some tests over the weekend and it was much sharper than my 24-105, 24-70 II, and 50 f/1.4 @ 50 mm.

BTW, I ordered two copies of the 50A to test side by side about 2 hrs apart in early April. I received one copy about two weeks ago and just received the second copy today. So, assuming it tests out, I may have a second copy someone could buy off me.