I wonder how this will stack up against the Sigma 50/1.4, which I hear is of superior design to the current Canon 50/1.4. The Sigma has a 77mm filter and is a BIG chunk of glass - I wonder if the new Canon 50/1.4 will be as big.
It should have no trouble beating the Sigma, since the existing 50mm f/1.4 beats the Sigma on FF cameras, but is better on crop cameras.http://www.photozone.de/canon-eos/522-sigma50f14eosff
The new Sigma AF 50mm f/1.4 EX DG HSM showed pretty impressive results in our APS-C review. However, it's not quite as convincing on full format cameras. The center quality of the lens is certainly fine and even great from f/2 onwards. Unfortunately the border & corner performance is rather disappointing. It's downright soft at large apertures and never really better than good even at the very best setting (f/8). The amount of vignetting is quite heavy at f/1.4 but that's rather typical for such lenses. If you stop down to f/2.8 this will not be a real issue anymore though. The lens produces moderate barrel distortions which is a tad more than the standard in this lens class. Lateral CAs are very well controlled and not really field relevant. The bokeh is quite decent although not perfect either.
A real highlight of the lens is certainly its build quality. Unlike most 50mm f/1.4 it features a metal body and a good focus ring. The HSM AF drive is very fast and virtually silent. Unfortunately the lens suffers from residual spherical aberrations (focus shifts) when stopping down so accurate focusing can be tricky.