Someone recently described Spore as more of a toy than a game. I think this is the best explanation possible. While it is fun, it is something that you play with more than just play. It sort of reminds me of software like Petz but with a bit more to do. Spore isn't hard to understand, although there is a lot of customization and variance (so far) in the critters you can make.
I am currently in the Tribal phase with some more to go. Tribal phase is definitely more challenging as there is more to do to control your tribe as you battle the other tribes for survival. However, it's still not really hard to play.
Seems like it is great for casual diversion rather than serious gaming, but you know, sometimes a light, casual diversion is perfect. I don't always want to fire up WoW and try to find something to do...either I invest hours to run instances or I mine...which is boring. This is a simple 15 minute or less/more investment that is fun in a pleasant way.