Studded tires are expensive, but if I lived in South Dakota I wouldn't think twice about buying them if I were (was? SadieKate, help me out here please) planning on riding in the winter.

I used them a handful of times when I lived in DC. When it snows and ices there, the Metros break down and the traffic comes to even more of a standstill, but we still have to get to work. The bike paths are not plowed or treated and turn into ice sheets. So I got studded tires and got to work before everyone else.