I like what Sigma has done and I absolutely love my 35mm 1.4 Art lens, but I just don't understand the rationale of making this particular lens.
Sure, the "list" price of this lens is lower, but with the amount of white boxed Canon 24-105's floating out on the market, this is a case where the street price of the OEM's product is going to be lower than that of the 3rd party manufacturer.
Put another way, most everyone is going to ALREADY have the OEM version of this lens. Sigma's version would have to be noticeably better for people to want to buy it when they already have the Canon. And the early returns suggest it isn't better at all.
I'd rather them put their energy into making a premium, Art version of the 50mm.