I really wasn't a fan of the biodome, with the exception of the fact that we were able to walk around in a rainforest when it was 7 degrees (F) outside.
The facility is small and crowded, packed with obnoxious tourists and parents with screaming kids. It's like Montreal's own little Disneyland in a dome. We walked all the way through it in any hour or so, then went back because we thought we had missed something (we didn't, it was just that small).
The zoo aspect was pretty depressing - the animals (when you could find them) were stuck in small pens and cages. The penguin exhibit was the worst - several dozen penguins stuck in a glass box smaller than my apartment.
If you've never been to a Biodome and you have some free time and an extra $10 (bring your *expired* student ID), it might be worth the trip, but I certainly won't be going back.