Have to say...this new Canon 35mm is a very mediocre lens, to me.
I know that a lot of people had problems with their Sigma 50 mm f/1.4...but I have had nothing but a great experience with mine...and I personally think that the bokeh is just incredible. As good as it gets with a 50mm f/1.4.
These new Sigma lenses (artist Series) appear to be a cut above anything the Co. has done in the past and reports are the customer service is on the mend,too. ...but don't tell what you are going to do. Show me. I think that Sigma has done that with their new 35mm....actually I think Canon has, too! :-)
The numbers don't show a very mediocre lens. To put things in perspective, look back at LensRentals' "The Great 50mm Shootout" —http://www.lensrentals.com/blog/2012/01/the-great-50mm-shootout
and it appears that the new Canon 35mm f/2 IS offers better resolution (center & average) wide open at f/2 than ANY of the 50mm lenses at f/2, with the exception of the legendary $4,000 Leica Summilux. And keep in mind that most of those 50mm lenses are stopped down at f/2, whereas this new 35mm is wide open.
It can't be too mediocre
if it offers better resolution at f/2 than any of these lenses do at f/2
Zeiss 50/1.4 (for Nikon & Canon)
Zeiss 50/2 Makro Planar (for Nikon & Canon)
Zeiss 50/2 Planar (for Leica M)
Zeiss 50/1.5 Sonnar (for Leica M)
Leica 50/2 Summicron
Leica 50/0.95 Noctilux
It's not surprising that the new 24/2.8 IS and 28/2.8 IS scored a bit higher on resoluton, as they are both f/2.8. Stop down the new 35/2 IS to f/2.8 and it will score higher too.
One thing that the Sigma proves is that to make a better lens for a DSLR, you often have to make it BIGGER. So it's bigger than the already largish Canon 35/1.4L. It's no doubt going to be great for usage at f/1.4, and thus very attractive for photographers who will make use of that. But for photographers who typically shoot at f/2 and smaller, the more compact Canon 35/2 IS may make more sense, especially with the addition of IS.
People are comparing the price of the new 35/2 IS and the old 35/2 and asking why pay so much more "just" for IS? — but they are overlooking the fact that the old 35/2 had that noisy AF motor from the 1980's and has a cheaper build overall. Also, keep in mind that the new 35/2 IS is being introduced at $850 but isn't going to cost $850 in the long run. The 24/2.8 IS and 28/2.8 IS were introduced at $850 and $800 and are both under $700 currently.