So I never know if I've walked the full "underground" city or not because I end up going up the stairs and being on the ground.
While it's a great path for cold Montreal winters, it's kind of easy to get lost. As for the stores, they aren't anything special. They are your regular retail stores.
I'm not sure it's worth a visit for tourists (especially in the summer) as I'd rather walk outside and see more people and get some sunshine.