Reactive: I just used this site's own lens comparison and came to an astonishing conclusion: stop down to f4 or smaller and you might as well buy Canon's 50mm f1.8. Try the comparison yourself. Above f4, the figures are near *identical* across the board, with just a slight barrel distortion on the Canon (easily correctable in software). So do I spend £850 on the 815g 100mm long Sigma, or spend £85 (yes 1/10 price) on the 130g 40mm long Canon? What could I do with the spare £765? New camera body? A new high-spec laptop to add a touch of sharpening to my Canon's images?
@Reactive I have the strange feeling that what you shoot and what the others who are commenting on your post shoot are not the same.
Let me point you to this link as my argument to why this lens is quite a valid contender for a good number of us: http://petapixel.com/2012/04/24/gear-doesnt-matter-except-when-it-does/
Let me also point you to SLRLounge's comparison test and say this: for closeup headshots or very detailed shots that are taken wide-open, adding "a touch of sharpening" is just not good enough for some of us; in fact, the lens you stated is worthless to me as the color rendition, sharpness, and flimsiness are horrible. I need to be happy with the images I take, and my clients need to be happy as well. http://www.slrlounge.com/head2head-sigma-50mm-f1-4-dg-hsm-art-vs-canon-50mm-f1-8
And, as this next link says, gear doesn't matter — except when it does: http://petapixel.com/2012/04/24/gear-doesnt-matter-except-when-it-does/
Mais78: The new Sigma lenses are very good optically and shine in lab tests. But as a owner of a Sigma 35mm f1.4 who went through 3 copies I can tell you that in real life the AF is hit and miss and certainly you cannot trust any focus point that is not the central one.
I have shot with the Sigma 35mm in plenty of focus critical situations, including at what felt like a pitch-black venue. It consistently nails focus to the point that I do not feel the need to snap more than one duplicate "just in case". This might be a YMMV issue, because for every time I hear a complaint about the Sigma's focus, I find another that has my experience.