I have lived in Pittsburgh for almost six years, and somehow never visited this zoo until a few weeks ago. My company was a sponsor for a fundraiser held here after-hours on a Sunday evening, and I attended with several other co-workers.
I was actually very impressed. The zoo was much larger than I had expected. After we we had finished our volunteering shift, we took some time to explore. I liked that the zoo was very easy to navigate and basically you just follow the path--it was easy to make sure that we didn't miss any of the exhibits. The aquarium was great as well, and also had a decent number of exhibits.
It seemed like there were plenty of amenities and places to grab a bite to eat. It would be nice to go back during normal hours (we were a little rushed since the event was going to end soon). Overall a good experience and one worth checking out!