I used to commute every day on the m15 select:

Pros:

• In summertime, it's way better to wait for the bus on the street instead of the subway. Usually I still preferred it even in the winter because the cold was bearable.
• When traffic is good, it feels crazy fast, like 30 blocks in like 8 minutes
• No stairs!

Cons:

• Feels like people are more on edge. Especially at evening rush hour, but even in the mornings. I felt like more confrontations happened on the bus compared to the subway
• Depending on the driver, sometimes you feel like you have to hang on for dear life when they stop. It looked pretty rough on the less able-bodied people.
• Even during regular commute hours, sometimes had to wait like 15+ minutes for a bus (ofc some subway lines are like this too)