When I travel to Montreal for business I have the pleasure of staying in a company residence. The only downside is that the residence is quite a distance from the office downtown. That being said, the Metro is right around the corner in walking distance.
The Metro in Montreal is very clean, with some pops of graffiti here and there. Its used heavily and if you hit it at morning or evening rush-hour you will be guaranteed to feel the pinch of standing room only.
The cost is so much better than taking cabs - believe me, that's what I did on my first trip, and have avoided it since! Our stop does not go all the way to the office so once you arrive in town you just use your metro pass to hop on a bus and voila you'll find yourself at the front door of the office lickity split!
My last trip gave me quite a bit of entertainment as the local pigeons lined up behind the yellow line along with us ... appearing to watch for an oncoming train! Too funny! (I grabbed a pic with the cell to prove it!)
I'm a fan of the STM - both the Metro and the bus line!