I've been to a LOT of zoos both in the US and internationally. The Pittsburgh Zoo is pretty mediocre. Parking was really out of the way in this creepy, dirt path, cramped lot. I'm not sure if we had to pay but just guessing that we did since you have to pay for parking everywhere in Pitt.
The aquarium is definitely better than the zoo. We can thank PPG for that! The zoo itself was very small and had very few animals. It wasn't very memorable and there were no scheduled shows. I have a rating system for admission zoos and free zoos. This zoo runs you at about $16 and doesn't even compare to the free Lincoln park zoo in Chicago. The exhibits weren't creative or expressive. I think the best part about the visit was just the entrance escalator since everything is on a freaking hill in this city.
The Cleveland zoo (free for residents) is about 3x better. It has that sweet rain forest dome that's separate from the zoo and an absolutely HUGE variety of animals and cool looking exhibits.
Going here once was quite enough. I don't really recommend it. Zoos i absolutely do recommend:
-Cleveland Metroparks (hey, only a 2 hr ride away for a better experience)
-Smithsonian DC Zoo (FREE!) Only about a 4 hr drive away