I've been to the Pittsburgh Zoo & Aquarium three times, in the spring, fall (most recent visit) and winter. Winter and fall are the best times. Some of the animals who prefer warm weather, like the komodo dragon, aren't around, but there is still plenty to see.
We saw the tigers born in 2006 a week or so after they were let out in the public areas, and then again this fall. That is one place we could stand and watch for a hour. Another is the polar bears, and with the new tunnel that goes under the tank, all the more fun.
Monkey house is rather sad. Seems too small for the animals. We only go in to warm up.
Not a huge fan of the aquarium. I've yet to go to an aquarium that matches Boston's.
During the cooler months there is always staff who are happy to chat about the animals, which is another reason I like going in fall and winter.