I was very cautious at first before buying another brand than Canon. I've owned a 35 f2, and found it too noisy. It was also front focusing a bit. I've been waiting and hoping one day Canon would release a new version (ef-s ?) of it. But nothing came. When I finally decided it was time to get a better medium range (eq 50mm) lens,I was tempted to exchange it with the Canon 28 1.8, but when I compared it to the Sigma, there was no contest: The Sigma was always spot on, perfect sharpness, while the 28 1.8 was a bit hit and miss.
Also the look of the Bokeh is great. Actually I'm not really convinced the 35 1.4 L can be that much better. With all the reviews I read about the focusing problems of the Sigma, I must tell I'm very surprised (and pleased) it is so good !!