Pretty fun and rewarding to work towards adding things to the zoo and upgrading each habitat. Wish there was more to it, like having a visitor satisfaction meter tied to, say, how well organized your zoo is or having more that you can do with the cash earned per level instead of only using it for power-up tools. After a while, it becomes too easy to accumulate money since there's so little on which to spend it. Also having the option to have friends and visit other players' zoos would be so fun.