Simple equation: rye+mustard+pastrami = deliciousness. And I agree that the black cherry soda puts it over the top. I don't ever drink soda, but there's something magical about smoked meat + cherry soda. I liken it to the friend or coworker who you thought was a health nut - whom you suddenly find bumming cigarettes off strangers at the club to have with their drinks... If your that person, you know what I mean. Again I say there's just something magical.
Many will also compare this place to Katz's deli, but this place does it right in terms of price as well as the meat to bread ratio of the sand which.
Long line, but well worth the wait if your visiting Montreal. Locals already know this.