I've visited the zoo several times, it's one of the better zoos I've been to.
The most recent visit was on a morning in May after it had rained overnight. We got there at opening and while there was a line to get in, it wasn't overly crowded until we were leaving.
We spent about 3-4 hours there, the animals were active, and the admission price was fair. The Pittsburgh Penguins mascot was there doing giveaways.
If you're in the area and are looking for something to do. You can't go wrong spending a day here.