The 747 Express bus service is fantastic and runs on a daily schedule between Pierre Trudeau International Airport and Montreal Centre-Ville. The L'occasionelle fare card that can be purchased at an automated machine in the airport runs for about $13 CAD per person for a weekend use card. Far cheaper than any cab.
The buses are perfectly fine and offer free WIFI. The stops are located nearby hotels in downtown.
Never understood why the excellent underground metro system was not extended all the way to the airport but the 747 bus service more than makes up for it.