Best way to get to downtown Montreal from Pierre Trudeau Airport is to take the 747 Express Bus!
Very good value. Better deal than taking a taxi from the airport to downtown.
$10 per person is what it will cost you. Our flight arrived from the U.S. around 5pm. After getting our baggage, we looked for information on the bus. Found the info we were looking after exiting the International Customs area. There was a information desk (close to the exit) with people to help us. They told us which stop to use which would bring us closest to our hotel. They then directed us to one of the machines which we used to pay for our tickets.
Took us about 45 minutes in rush hour to get to downtown Montreal from the airport. Would have taken us much longer if we used a taxi or rented a car. There is even a special lane just for this bus which made the trip faster.
Chances of using this service in the future is very high! Great value!! Would not hesitate to use them again.