Really, I would give this place 3.5 stars. It is a great, relaxing place to spend a cool day. DO NOT go when it is hot - you will be miserable and so are the animals. Overall, this is a pretty standard, older zoo. What distinguishes it, though, is the amazing desert backdrops. The best areas of the zoo are those that reflect the local landscape, especially the big horn sheep area dramatically set against Papago Peak.
As an aside, the kettle corn made on site is really good as well. If you want a salty-sweet snack, this is the way to go.