Here's what I've been waiting for - a 50L to 50A comparison from SLR Lounge
I think it's enough to convince me to cancel my pre-order. The Sigma is sharper, but not shockingly so, and the Canon's bokeh is slightly better (IMHO). The Canon also appears to have ever-so-slightly better contrast, while the Sigma has better CA control, but again, only by a hair.
The other thing I've learned is that you'll have buy the USB dock to enable full time manual focus (which I guess isn't standard for Sigmas). That's crappy.
The 50L has killer build quality and USM in a much smaller package and I don't think the Sigma is worth 950 of my dollars for such subtle differences at f/1.4 in what for me, is a portrait lens.
I guess I can't cancel it till the 24th, so I'll keep my mind open until then, but I think I'm going to cancel and resume the 50L II vigil
The differences are little, very little. I can understand while you own the 50L not spending money on the Sigma. I would not spend 500 euro more (here in the Netherlands) for the 50L while the difference with the Sigma is so little. So as a starter for the 50mm, my choice would be the Sigma.