This site is incredibly beautiful and huge! The botanical gardens are well worth a visit if you are in or near Montreal. The Chinese Garden was particularly captivating. Plus, while you are there, you can wander across the street to check out the Olympic Park.
My advise: Plan to spend a lot of time here. There is a lot to see and you do not want to be rushed. There is a decent amount of cultural information presented in addition to gardens. Definitely wear comfortable shoes.
You need to pay to see the indoor gardens. They were neat, but perhaps unnecessary, as the outdoor gardens were so extensive. I am unsure whether or not you need to pay to go outside. Our tickets were not checked as we went in, the information on the website is inconclusive, and another reviewer mentioned outside being free. If you are planning a trip here, I would inquire about what the ticket price covers before paying.