If Sigma can maintain this level of quality and price across more lenses, Canon might have to reconsider its pricing. Canon's near monopoly on the high end AF lens market might be coming to an end finally. Of course, this could also be a "loss leader" or gouge from Sigma. Time will tell.
Their 85mm is just as good, so this isn't a brand new thing. I'm glad to see them competing not only on price but also on quality. I still think they have a significant hurdle to overcome before Canon has to worry. Their still seems to be an issue of people being leery of third party lenses due to uncertainty and hearse concerning problems. A good PR campaign and maybe a loaner program would go a long way.
6D, Fuji X-E1
Tamron 24-70 2.8 VC, Canon 50mm f/1.2L, 70-200 f/2.8L IS II, 400mm f/2.8L II, 100mm L IS Macro, Sigma 85mm, & 35mm f/1.4's, Rokinon 14mm f/2.8, Canon 2x Extender II, Kenko 1.4x, 430 EX II, Elinchroms