I would normally give this zoo three stars, but I'm giving it four because it's a free zoo. Some of the enclosures are outdated, yes, but the zoo is constantly working on improving the animal spaces. It's amazing that they are able to have what they do for a free zoo. It's a great place to hang out on a nice day when you have nothing else to do. It's fun to take a picnic with to picnic in the park next door. True, the parking stinks, but we're generally able to find parking after driving around for a little while.
For those commenting on charging admission: what I was always told when I was growing up in Madison is that the zoo is never allowed to charge admission. The Vilas family stipulated that the zoo need to stay free. If the zoo were to start charging admission, then the land would go back to the Vilas family. Then there is no more zoo for anyone to enjoy, good or bad.