Ya it's a different kind of game. You basically get to create your own species and customize every aspect of that species. Are you going to be a friendly species or are you going to try and takeover the Galaxy?
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.
My problem with Spore is that it's a real 'geek game' on the level of MMO's like WoW etc. It's the kind of game you can spend months on (of accumulated game time) without any real ending. Also, since becoming a console gamer, I've come to resent PC games by and large - a purely personal choice but I figure that PC's are so much more diverse, I'd rather spend my gaming time on a machine built for gaming (ala, console) and my computing time on a machine built for computing (ala, computer).
I'm a computer gaming geek at heart and I don't see that ever changing. Having spent the money on a rig that allows my games to look better on my computer at 1920x1200 on a 24 inch monitor with all the settings maxed than they would on a console, I like to exercise its ability. The only gaming consoles I've ever owned in my entire life are a NeoGeo (if you can call that a console) and now a Wii, which I really only play MarioKart and SmashBros on. And that's more for just passing some time with the wifey or goofing around with when friends are over for a cookout or something.
Beff, comparing Spore to WoW is a big stretch. I've played Spore for less than a week and I'm almost at the "end". And yes there is an end. WoW on the other hand, even though it does have a definitive ending, would take months if not years to get to. And that would require being in one of the top guilds in the world.