You evolve from a single-cell organizm to eventually a space-faring race. Your vehicles and buildings are completely customizable. You could really spend a lot of time just on deciding how your civilization was going to look. There's a LOT of room for customization if that's the kind of thing that interests you. I haven't done a whole lot with the space phase yet but so far it's pretty cool.
If you've got a bunch of other games that are taking up your time, I probably wouldn't recommend Spore unless designing your own species is something that sounds interesting to you. It is the type of game that you can just pick up and play whenever though.
Hope that helped.