Definitely a must-visit if it's your first time in Montreal. This was built for the '72 Olympics and is attached to the Stade de Montreal next door. The admission for the funicular/tower is separate but you can buy a combo ticket if you go into the Stade first.
This tower offers a 360 view of the city of Montreal. I appreciated the information available under the windows to let you know what you're looking at. We went on a kind of cloudy day so it was a bit hard to see everything, especially Mont Royal (which is probably the best thing to look at).
It takes a bit of time to get up here and get back down if there's a lot of people so prepare to wait a bit. There's also not really good air conditioning in the waiting areas so it's a bit hot in the summer months. There's a public bike station on the property so it's a great spot to bike to/from if you're on a tour of the city.