I was referred to Merson's by a friend and now it is the only place I'll go for tire related purchases in Montreal.
The people that run the joint speak great english, and are friendly to english speakers, which is why they continue to get my business. Additionally the guy that sold me the tires was very knowledgeable about the products he was selling. I also got a good deal on tires here, 4 yokahama's for $600 out the door...compared to 4 goodyears from canadian tire another friend got for $540. Also compare my purchase to $100 per tire on tirerack.com, with installation and Canadian taxes included in my purchase, $600 is pretty damn good. Conveniently they will also store your winter/summer tires for the season at a rate of $16 per tire.
Be sure to make an appointment or you'll be sitting for several hours.