I was bored enough the other day that I decided I wanted to try out this game. I gotta say, there are an awful lot of negative reviews out there on it so I was a bit nervous that I wasted my money. I've been addicted to it! There are so many stages of life to play with, you can start out with your species as an embryo and take them all the way to evolving so much they can travel in space!
It's not a game you can beat overnight because the stages take a while, it involves either becoming allies with other species on the planet (You can select any one of the multiple planets they provide) or conquering them into extinction. If you connect your game to the internet you can download other people's creations, houses, city halls, creatures, vehicles, land vehicles and others, tribes, and much much more. You also get to visit other users' species on the planet instead of just maxis defaults. But even with Maxis defaults, you will probably never play the game and start a new species, and run into the same species twice, there are thousands. The level of customization in-game is so amazing that your creations will never look like someone else's.
Have fun, you'll be addicted!