Quick and easy, I like the STM.
While I have visited Montreal on many occasions, it was my first time using the metro system. I didn't have the opportunity to use any of the buses but I found the metro (aka the subway) making it easy to get to the places I wanted to go. It was also ideal that one of the major connections, Berri-UQAM, was located underneath the bus terminal.
I found the stations and metro cars to be clean and safe. Plus it's cheap. It's $2.75 per an adult ride, the same price as Toronto, but if you bought a weekly pass, it's way cheaper than TTC's weekly pass. Also it's more flexible regarding a traveler's schedule.
All in all, the STM is one of the better metro systems we have in Canada. It made it simple for anyone who isn't familiar with Montreal to move around with ease.