The Pittsburgh Zoo has been on a slow-but-steady course of improvement since I was a kid and all the animals were in tiny enclosures. There are still parts of the zoo that seem inadequate, such as the Tropical Forest - I felt really sorry for the orangutan. But overall, if you like zoos, this is a nice one, plus it has a good aquarium in the middle.
We haven't visited this zoo for a number of years because the last time we tried, it seemed like my daughter couldn't see anything for the handrails. There is still some of that going on, but it seems like enough of the exhibits have been redesigned (and maybe, also, it's that my daughter is taller :-) ) that it wasn't the issue that I remember it to be. I'm taking one star off because it was still a problem in some places. Also, be aware that the zoo is built on a hill (well, it's Pittsburgh...) and the slope can be nasty at times.