First off - I found this place bloody difficult to find! Maybe I just didn't understand the concept but I thought that I'd be able to zip down after a visit to the museum and at the point on my map where it said there was an entrance, I'd be able to pop underground. Well, I spent nearly 2 hours looking for the ruddy place! I was finally headed back to the hotel when, by lucky chance, I found an entrance! Yay!!!
I was more interested in the Underground City as, well, an underground city than I was interested in it as a mall. The concept fascinates me and I wanted to see what it was like. But unfortunately, once you're in it you forget that you're underground and just feel like you're in a mall. The concept still amazes me, and I'm glad that I checked it out (and I think that you should too if you're in Montreal) but I'm not sure if it would be someplace I'd make a point in putting on my itinerary again.
I can definitely see this being a benefit in the winter especially if you could use it as part of your commute so you could walk without having to trudge through the slush and freeze your toesies off!