The simple awesomeness of penguins, monkeys and a kapibara in Canada warrants a visit to the Biodome.
90 minutes or two hours strolling through the Biodome's ecosystems are time well spent indeed. The relative freedom of the animals to roam their pavilion is a great change from zoo cages, and you can play monkey-spotting or admire the beautiful macaws, knowing too well you can't own one because it would out-live you.
Try to avoid school holidays and weekends if possible. You don't want the place to be bustling with hyperactive kids having a great time. You also definitely want a spot for sitting in the penguin section and a view on the swimming penguins, without a crowd of munchkins obstructing the water line.
Admission is steep if you aren't a Quebecer or a student. Definitely worth a visit though. If you bundle the Biodome with the Botanical Garden the deal gets better.
Do not do the Olympic tower. You get a better view from the Mont Royal chalet. It is free and involves a healthy trek up the hill.