I used to really dislike the STM but having experienced other public transit systems elsewhere I now know that we're spoiled here in Montreal. Yes there are service interruptions and buses sometimes fail to show up but overall it does a good job of getting you where you need to go. I work downtown and if I were to drive not only would it be more expensive but it would also be longer because I'd be stuck in traffic. Instead I hop on the metro and spend my commute reading and listening to music.
The trains and buses are generally clean by transit standards and in more central areas they come frequently especially during rush hour. Even during non-peak hours the trains come often enough. Out in the suburbs bus schedules aren't as good but that is understandable. The STM also operates the 747 bus service out to the airport which is significantly cheaper than taking a cab.
My one major beef is with the anti-anglophone attitude some employees have and the sometimes lacklustre customer service. I've encountered some really rude employees over the years.
They offer a variety of fares so you can pick the option that best suits you. There are monthly, weekly, 3 day and day passes. You can also purchase single use tickets as well.