I've still got a whole expansion pack left to play, so I don't want to get too retrospective here. I'll just say that I didn't get a lot of use out of the Creepy and Cute Creature pack, probably because I didn't want to hang around in the Creature Stage for the hours and hours it would take to unlock all the parts. What I might do for Galactic Adventures is just skip straight to the Space Stage and then play around with the whole palette to design my species.
I'm feeling pretty good here at the halfway point. I could easily keep playing my current galaxy for another ten hours, at least (I'm just a couple of badges away from getting the highest possible title and I haven't even come close to the center of the galaxy).
Yet starting over from scratch does have its appeal. Even though the middle stages kind of suck, the whole thing taken together is a wonderful process of discovery. As you progress through the stages you create the story of your people, the way their evolutionary history influences their behavior, the cultural baggage that shapes their technology, their approach to meeting strangers in the vast expanse of space.
Technically, this isn't necessary. You could easily be arbitrary with your choices, and when it comes to designing buildings and vehicles, you could just load a preconstruct and be done with it. But I find that when I'm playing, I can't help myself. Whenever I tweak my species in the creature stage, I always ask myself "what evolutionary pressure pushed for this change?" Or when I design a vehicle in the civilization stage, I wonder how it expresses my peoples' ideology.
It may be that the reason I like Spore as much as I do is because I bring this strictly unnecessary speculation into it. It makes me feel like the world is broader and deeper than it really is. If that's the case, I'm not really playing the game, I'm playing a different game I made up myself that uses Spore as a starting point but then goes off in its own direction.
Or maybe I'm overestimating the power of my imagination and this experience is exactly what the designers intended when they made the game.
Either way, I don't think it matters. I love Spore and I have no problem playing it for another 20 hours.