A great, and affordable, way to see the city the way the locals do. Montreal has to be the most bike-friendly city I've ever been to. Plus, you can work off all that sandwich you ate at Schwartz's!
You may need a local to show you around a bit the first time or two. Remember to give the bike you want a once-over before taking it out of the rack.
If you have a smartphone, I would highly recommend getting an app that shows all the BIXI stations on a map. There are even ones that will show you how full/empty stations are.... which is extremely helpful at times.
Highly recommend!