I have a long history shooting with Canon EF 50 F1.4. I mostly use it for concerts, when I am close to the stage. When I purchased Sigma I was looking for better focus accuracy and better sharpness/contrast wide-open (as per glamorous review on DPReview).
The copy that I bought was back-focusing heavily. I had to set micro-adjustment value to +12, but I still wasn't quite pleased with the result. So I sent the lens to Sigma and they calibrated it. The focus problem is gone, but other problems remain:
1. My Canon 50mm has better sharpness in the center (I don't care about corner sharpness - it is bokeh most of the time)
2. There is no advantage in focus accuracy. Both lens miss focus at the same rate. I would actually say that Canon performed marginally better. I get more keepers with Canon.
3. Bokeh is better than Canon's, but the green-purple fringing in out-of-focus area is really evident and looks quite ugly to me. The pictures have this purple haze in the bokeh which changes the tone of the whole photo. I don't like it at all.
In essence, the increase in size and weight is not justified by any performance improvement. The lens has a distinct character that I don't like. The lens may look good on the charts and test patterns, but in real life it is rather disappointing.
I took some test pictures of the same subjects with both Canon and Sigma, changing them randomly. After that I looked at all pictures without knowing which one was taken with which lens. In this "blind" test, both me and my friend would choose Canon over Sigma every single time. There is simply no edge in Sigma whatsoever even after calibration.
It goes on Craigslist tonight.