For the record, I disagree with the "Sigma is clinical" assessment. I've seen a lot of images that look wonderful from that lens. I lean towards the Canon mostly because they both look great and the price difference is significant.
From some of the images that I have seen posted on the web the out of focus or 'bokeh' of the Sigma has, on first impression, looked really good. However I think that the transition from in and out of focus is quite abrupt, at least when compared with the 35L. That lens has a proper ground glass aspherical element, and I do find that on lenses when this is used there is a more 'glassy' or 'liquid' quality to the out of focus area. I'm guessing that the Sigma is pretty highly corrected for chromatic aberration to get the sharpness, and I'm sure it doesn't use a ground glass aspherical element, and the out of focus is a little more 'plasticy'. ( I'm being really scientific here).
I wonder if this is what Dustin is relating to ?
The EF 35 IS will use a moulded element, but as with the other Canon mid range primes the bokeh transition is good. Pretty clever stuff for such a sharp lens.
Have a look at these images, and tell me this lens has a smooth transition zone. Notice how rough the highlights look, both in front of the focus plane, and behind it.
Frankly the 40mm f/2.8 pancake, had as smooth of a bokeh as this, if not slightly superior, it seems to me. The Sigma 35mm Art, appears to also have a smoother transition zone in the bokeh, than the Canon 35 f/2 IS. Just have a look at Photozone's samples from the Sigma. They do fault it as having "slightly busy" bokeh background, but very smooth in the foreground. But its background highlights, do not seem to have such a pronounced "bright ring" around them, as does the 35 f/2 IS. It would help if the exact same scene were photographed to compare, of course.
I see the IS feature as the only very good aspect of the Canon 35 f/2 IS lens, going by these samples. Certainly a nice feature to have. Also I will admit that for bokeh smoothness at 35mm, the Canon f/1.4L is still clearly king. Also, if I had to guess which had the smoother background bokeh, between say the Sigma 35mm Art, and the Canon 24-70 f/2.8 ii (at 35mm), the Sigma might edge it out a bit. Would be interesting to see a direct comparison of that. If there are no highlights in the background, then all of these lenses can probably portray a relatively smooth blur, as most lenses can.
I use photozone a great deal for comparing the resolution of lenses, but to be quite honest I never look at their samples. However I've just looked now at your prompt and it's confirmed my belief; the Sigma samples are pretty useless for a bokeh assessment.
Have a look at the ones in the CR thread:
As I said in my earlier post, my initial reaction was this looks really good, but then over time I thought they are just a little - 'clinical' - is that the word Dustin used ? From what I read Dustin has come to that conclusion over time, which makes it more pertinent, and I'm with him on that one.
Also I mentioned earlier that the likes of the 35L, 50L and 24-70 II use a much more expensive lens manufacturing process. Mid range lenses such as the 35 IS etc. use moulded glass elements, and really cheap lenses use plastic ones.
However I agree with you on the 40 pancake - and remember you are talking to someone who ditched his 35L for the pancake ( But don't tell anyone else that !). I don't have a use for a really fast 35, I'd much rather use a 50 or 85.
Overall I'm very tempted by the 35 IS, not for it's aperture but because it has IS and is really good across the frame when stopped down.