The STM is a superb subway system. It is quiet, clean, well run, well staffed, efficient and cost effective. Although it stacks up favourably to most of the worlds transit systems, for the purposes of this review, I'd like to compare it to the Toronto system - The TTC.
Coverage: The STM casts a wide net over Montreal. Considerably wider than that of the TTC.
The Trains: Although the cars are somewhat smaller they are invariably cleaner, better lit and offer a much smoother ride. They also manage NOT to damage your eardrums every time they stop in a station.
The Stations: Even the rattiest STM station is still pretty OK and they are mostly kind of well, grand. Pretty much the opposite of the whole Soviet surplus vibe going on in the TTC
Fares: $3 a fare for a one shot deal for both systems - However, the day pass on the STM is $8 compared to $10 for the TTC. The STM shines a bit brighter in terms of offering a 3 day pass for $16 and a prepaid card system for both regular riders and frequent visitors.
So - pretty much across the board - advantage STM