This won't be the kind of zoo that will win international awards (although in cases with things like 'zoos', there's always an award from some society to hand out), but its a great way to spend a long morning and early afternoon with students. In this particular case, as chaperon for part of my gf's 2nd grade class.
Something about ushering a group of five hyped up 7-year-olds through the chalky landscape, bomb-pops in hand, loaded up to their D-Backs caps in sugar and expectations can make any place special. For these kids, the Phoenix Zoo delivered, and in turn delivered on my cautious optimism as well. A saccharine start in the "Tropics Trail" with a very smooth finish at the tigers, left every single one of my kids-for-the-day overenthused and buzzing.
The highlight was their interactive venture into the Sting Ray pool, where for $3 (trying to remember) they got to dip their hands into the water and "play" with the rays. This made them literally goofy.
I was also surprised that their food court was fairly extensive and the quality of food far exceeded my expectations. I had a BBQ burger (comes with an onion ring on top) that was actually top-tier fast food. I remember it being modestly priced too (in terms of the typical amusement park racket).
Overall, I was highly impressed with the Phoenix Zoo. As others have mentioned, the San Diego Zoo is the highest standard deviation from the mean as far as zoos are concerned, but I can certainly call the Phoenix Zoo a benchmark on which to build.