Unlike zoos in the USA, you can get very close to the animals here instead of observing from off in the distance. We went on a weekday and the place was practically deserted. The staff even left the entry gate unattended as they socialized in the gift shop since they weren't really expecting anyone to come.
Be sure to take insect repellant; there are lots of biting insects and I had at least 15 bites on my arms. This is a nice escape from Quito, since the altitude is lower and it's easier to breathe. It's also warm and dry.
The zoo is located 28 kilometers north of Quito, off the Panamericana highway, on the outskirts of Guayllabamba. You have to take a bus from Quito for about $.50 and ask them to let you off at the stop closest to the entrance of the zoo.
My tour guide, Francisco Guayasamin, took me here, and unless you speak Spanish very well, it would be better to have a native with you, just to make things easier. I thought this would be a place for kids, but I really ended up enjoying myself. The zoo isn't huge, but it has a wide variety of species. A great website to find out more is here.