I live in NY but go to Montreal often because is the closest big city. Depending on where the american dollar is compared to the canadian, it may or may not be worth it. If I pay cash I change money to Canadian before I go. If you pay with credit card, the charges are changed to the currency where the card is issued and if the canadian dollar is weaker, the conversion is to the disadvantage of canadians. Like mamagus said in the first post, if she paid $2.50 in US, her credit card makes sure that she pays what is worth in canadian, which may be $3.00 depending on the day.
I like to use cash when the rate is the same.
Maria