I go either way on Zoos. They can be quite depressing- some of the higher mammals seethe boredom and have a dullness or a madness to their eyes.
Still, it's entertaining people watching. Much more so than watching the tigers sleep. It's a good chance to get photos of animals that are elusive or continents away, to play with different lenses and fiddle with camera settings that you wouldn't be afforded in the wild. Go on a weekday early and you'll have the cages to yourself, and the plexiglass will have a bit fewer smudges on it. I remember taking a day off from work to get to the Bronx zoo on a weekday morning while school was in session, and having a nice quiet stroll around and even getting to see a redtail hawk eviscerate a pigeon. Nature at work.