I'm certainly intrigued by the optical quality of the Sigma 35. However, I have the 35L now and my copy, at least, is razor sharp at f/1.4 and gets ridiculously sharp from there. The colors, AF and rendering are also amazing. No complaints there.
The main reason I'm interested in the 35/2 is the smaller size and weight. The 35L isn't huge, but it's not compact either. If I can get the same or even better optical quality in a more compact package, I'd probably be willing to sacrifice the extra stop. However, I shoot almost exclusively portraits (of my 16-month old daughter, who never stops moving), so IS doesn't help me at all; f/1.4 is a lot more useful in those circumstances.
The Sigma is bigger and heavier than the 35L (660g vs. 580g, and 3.03 x 3.7 vs. 3.1 x 3.4), so even if it's a little sharper than the 35L, I'm not interested. My 35L is plenty sharp, and the size and weight are more important to me.
I have the 40/2.8 too, which is incredible especially considering its size. In fact, I also prefer the 40mm focal length to 35mm. I wish Canon made a 40/1.8. I'd be all over that. I used to shoot Pentax and their 43/1.9 was my favorite lens ever.