Spore will be a poor game in the same way that The Sims is a poor game. And obviously I don't mean poor in the economic sense, since The Sims is the most successful video game of all time. However, as a game, it's pretty poor.
Ok, even that isn't very accurate. I guess I should say why Spore won't be very fun. It'll be engaging, interesting, technically incredible maybe even enjoyable, but it's not going to be fun. And this stems from two very important reasons.
One: there's too many game types. There are 4 or 5 stages of life to the game, with the first segment starting off as a Snake-like minigame where you're just floating around and eating things. Each stage is a different kind of game. Meaning it's 5 games in one! AWESOME!... NOT! The best games don't revolve around a bunch of small games, the best games revolve around a core concept that is honed and refined to perfection that it becomes a joy to pickup the controller and play in the world. When you mash a bunch of different games together like, not a single one will be that good. They'll be good enough to get by, but that's it. Jack of all trades and Master of none.
Two: there's no point. This is where The Sims comes into play. There's no real point with The Sims either, or other kinds of simulation games. That's what they are, they're simulations, you are presented with a model of some system and adjust various parameters and things and watch how the system reacts. And that's fine. That's fine as an interactive experience. I'm the kind of person that will play with a physics engine for hours at a time. But it gets boring. Where The Sims gained mass attraction is in the MySpace and YouTube like social aspects where you're creating and managing a virtual doll and expressing yourself. People would create custom clothes, animations, etc all as a way to express themselves.
Spore will be the same.
At the beginning it'll be enjoyable to see the impressive technology that has gone into the game. And it is amazing technology, there's no doubt about that. But once the gee-whiz factor is gone you're left with the mundane task of content creation.
In terms of content creation it's going to be the best thing available in the world. You're making these little guys and civilizations, creating their look, how they move. And the seamless nature by which people's creations cross-pollinate is innovative as well. But ultimately you're still left with the same mundane task, which I imagine will become quite boring not after long for a large part of people.
It's just like any social networking website. You join, there's the initial high of creating your profile page, and going to see what your friends have done, and then branching out from there. But after a week, a month, whatever, everybody's page starts to look the same and you wake up one day not knowing where you are wondering how you hit rock bottom.
In conclusion: don't play Spore or you'll get hooked on crack.