It's a great game. But $20 is a little bit pushing it.
Well, there are tons of sales, and as I'm writing this there's one right now, so you might want to wait for one.
Evolving you creature is something I found super fun. The graphics are really nice and the animals are cute....most of the time...
I also love the feature that your behavior effects your reputation as a species, you might be a vicious killer, or a very social creature that likes friends.
Unfortunatly, personally I find the extreme change of gamplay a little boring. It starts off as you evolving, killing or befriending other creatures, and eating them or plants. Then it locks the way you look and that's kind of sad, transforming it into a full on tactical birds-eye view of your planet, defending off enemies, and eventually going into..space?
So I see what to developers were doing, a little cell in the ocean to advanced beings in space, but it just didn't seem all that interesting to me at that point.
Depending on what you like, it has an ok replayibility level, but I like going into create and making my own cool animals.