The Metro (subway) here covers quite a bit of ground in the core Montreal area and is fairly efficient. I was able to travel to Little Italy, Old Montreal, Chinatown and Lachine Canal using this method of transportation and is much cheaper than a cab. I believe it was $10 or $11 for a day pass and is quite worth it if you decide to traverse different areas of the cities.
I liked the fact that they showed each stop electronically and helps compensate for any French language deficiencies you may have. My only issue with the STM is that it is a bit hard to navigate to the Metro itself because sometimes you have to underground 2-3 times. However once you are there, its quite easy
Overall, while one of the smaller transit systems for a metro area in North America, the Metro/STM is quite well run. .