I had 2 Sigma 35mm and returned both of them. I just couldn't get accurate AF in daylight and inside under fluorescent/yellow light. I used USB dock to calibrate it, and I noticed that during the day, the distance between 0.3-0.6 wouldn't always focus right. Plus if I used outer points, it would always hunt. I noticed even with center point pointing at something dark, it would hesitate to focus at first.
I really loved the sharpness, bokeh and 3D effect when it focused properly. The build quality is amazing too. Yes it is a little on the heavy side. If it would of always focused properly inside the house and outside, I would of kept in a heartbeat.
With my 100L, I had no problem at all, same lighting conditions. The focus was a lot faster too, like some pointed out (if used right). It does hunt however, when taking macro shots, unless you manually pre-focus, then it will be fine.
Now I really want to try Canon 35mm f/2 but I still think it cost too much. If it was $400 I would get it without thinking, but at $600 I would think twice.
Waiting for Canon 35mm f/1.4II to come out so the prices could drop a little