We went to Budapest Zoo at the last day of our trip and did not expect a lot (planned to spend there about 2 hours). But we were so impressed by the Zoo that we spend a lot of time there and were really satisfied (although it was in the middle of winter).
The Zoo is rather big, has a lot of animals and related information (like their exploration center). We have visited other zoos in Europe and can say that Budapest zoo is in the same row with the most famous ones (like e.g. in Prague and Berlin).
For sure the zoo visit takes some time (I would say not less than 4 hours). So it is a good choice if your Budapest trip is not very intensive and you like zoos.