Pretty nice sandbox game. I like vivariums in real life so I had to try it.
I think it works great for the most part, you can add your critters and watch them interact with their closed habitat and each other. As always, I find these small organisms really cute.
Perhaps some of them are a little redundant at the beginning since most of the early game creatures effectively occupy the same niche until you can afford a different organism later on, but that's a minor issue.
Also, I find it sort of amusing that only the mighty pomanatee, peerless firstborn of Poseidon, can truly threaten the closed ecosystem by uprooting plants.