I visited the Phoenix Zoo the other day and I really enjoyed my time here. There are four trails with different themes for each trail. The Arizona trail has animals native to the Southwestern United States, the Children's trail has animals that can be seen up close, the Africa trail has the rhino/giraffes, and the Tropics trail has monkeys and the elephant.
I feel as though the Phoenix Zoo does a great job of combining the confinement/borders of a zoo with the natural landscape of the area. It doesn't feel as manufactured/sterile as the San Diego zoo, for example. It feels as though you can get rather close to the animals.
One of my favorite exhibits is The Monkey Village - which is an enclosure with 14 squirrel monkeys that you can walk through and see them up close. They are just the cutest!
General admission is $17 with a student discount, but it's worth it to get a membership if you're a local and plan on going a couple times a year.
Overall, you could get through the whole zoo in 3 hours, but you can also slow down your experience and have lunch/visit the souvenir shop. I'm a fan of this place and definitely recommend it, although I think I would refrain from visiting anytime after April-October, since the Phoenix heat is just too intense!