I ride a bianchi volpe for commuting and touring year round in Ann Arbor, Michigan. I've never converted from my 27 speed setup in the winter. I did run into some problems with my cables freezing up in sub-20 degree temperatures, but they continued to work well enough most of the time.
Last year I used studded 35c cyclocross/mtb tires during the winter. They came in handy for a couple of weeks last year, but I found them to be a bit excessive for the conditions 2/3 of the time. I've talked to several people from Ontario and Alberta that use relatively narrow, non-studded tires in the winter. They claim that the narrow tires work better for cutting through snow, rather than wide, nobby mtb tires that tend to ride on top of the snow. But studded tires certainly offer an advantage in icy conditions.
I'm planning on going back to the studded nobbies this winter. What would be ideal would be to have two sets of wheels, one with studded road tires for the rare days when the snow accumulation is bad enough to warrent them, and one with regular road slicks for the rest of the winter. Alas, I only have one set of wheels and don't intend to blow money to buy a second set, so I'll be riding strictly on studded nobbies from Dec-Februrary.