I've just played spore into the start of the space stage, I really like it. But it's quite unlike most other games. I have a few friends that've played spore and told me it was shallow. Even one extreme who said it was down right boring. But when I asked them why this is most of them couldn't give me more then general arguments on the lines of "not enough to do" or "not enough of a challenge" which to me sounds exactly like the critique the Sims recieved from the gamer community and it still went on to become the biggest franchise in the history of the industry.
My own oppinion is that Spore, as far as basic mechanics go, is pretty shallow. Where in the Sims you had an abundance of choices Spore offers at best about 4. But here is the tricky part. Spore has a deep meta gameplay.
The Sims was quickly hailed to be a great way of just pushing around a doll house. That is, the game itself was really more about looking in on and affecting the lives of your sims then it was about controlling them. And the same is true about Spore. Spore is a tamagotchi for an entire evolutionary tree of a species from baceria to space aliens. But that's all it is. Spore is not a classic game where you rank higher then others and so on. Spore gives the player unending amounts of creatures and vehicles and stuff, and basically says: go explore. Poke around, see how they interact. This is real hobbyist entertainment, I could spend countless hours with Spore if I had the time. Just like I can spend coutless hours painting Warhammer figures, or countless hours playing with a dog that I don't have time to have.
Spore is in essence, a simulator that lets you poke and play with simplyfied evolution, don't expect a game. This is an experience. And it is as close to a work of art as I have ever seen.