If you hate nature and walking around for long periods of time this is not the place for you, otherwise it's a must see when visiting this beautiful city. The Montreal Botanical Gardens is HUGE so if you want to see everything expect to be there for about six hours. I am not exaggerating.
My friend and I went last year during in the early fall when they had all the lanterns in the Chinese Gardens. It was absolutely beautiful. The Chinese Garden's themselves is quite a sight to see as well, and it is quite spacious.
Another thing we enjoyed were the vegetable gardens. There was literally every kind of food that comes from the ground being grown there: vegetables, herbs, fruits... we were tempted to swipe an onion to include with our dinner feast later that night but there are signs all over telling us that they take vegetable theft very seriously so we ended up not doing it.
I recommend that anyone visiting (or living in) Montreal go check this out at least once, as kit was one of the more pleasurable non-nightlife experiences on my trip.
*Tip* Wear comfortable shoes. You will end up being there for much longer than you planned and there is A LOT of walking involved.