What, specifically, is 'weak' about the 50mm f/1.2L?
I'm guessing it is the relative lack of sharpness in border regions on full frame, from a quick glance at Photozone results. Seems to fall off significantly there. Not an issue on crop sensors. It doesn't sound like an issue in real world shooting, since if you're getting the f/1.2, you're wanting the current shallowest DoF at that length and unlikely to get everything sharp in frame anyway. There is the even older f/1.0 it replaced that's even softer.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 15-85, TS-E 24, MP-E 65, Zeiss 50/2 macro, Sigma 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye