Oooooh yes, this is one of the good ones. IMO, the State Street area has always been kind of lacking in non-food cart, non-sit down/dine in snack places. There are some, and it's expanding, but not much else as you move toward the Square.
Enter Mad City Frites. As others have said, they only do one thing - but they do it well. If you're looking for hot, perfectly textured fries with delicious dipping sauce or some good poutine, get here. Plus, cool design stuff: the tables have little holes in them for the fry cones!