One of the big questions I wonder about with Spore is, how would it be different if it had been developed and produced by a small, relatively unknown company? This may be moot, because the technical challenges of getting the creatures, in particular, to animate properly might well be too much for a small studio. Still, I get the sense that if that scenario had played out, Spore might be regarded as a hidden gem: a commercial failure, perhaps, but an interesting game doing things technologically and conceptually that most games do not.
All of this is leading up to my opinion that Spore isn't a great game, but I like it anyway.
To start with the bad parts, the creature stage. It's interesting, once. After that, it becomes boring as hell, as collecting all of the parts, not to mention points, you need to proceed takes a lot of time and exploration. All while you inevitably die multiple times when you run into some variation of an UberCrushMonster.
The civilization stage also deserves a mention here, not so much due to gameplay (which is way too easy on normal difficulty), but rather to the beginning of the stage, when the game immediately asks you to design 5-6 buildings/vehicles right away. While I sometimes enjoy playing with the design tools, doing one project at a time is definitely enough for me. I've played enough now that I usually pick designs from a list, especially for the vehicles, which I know I will barely look at, and won't have any impact beyond that section.
Another frustrating thing is that the game does award me achievements as I do various things, but as far as I can tell there is no place to view the achievements you've collected, or see what you need to do collect others.
My major complaint from the space stage is about the Grox. Once I realized the extent of their empire for the first time, I quit playing for a while. Doing anything to them would require a merciless grind of the worst sort, and at this point all of the races that I bring up to the space stage ignore the eventual order from their homeworld to go find the Grox.
The last complaint that springs to mind is about the Computer AI. While this is true of a lot of games, the computer empires in Spore do a terrible job of thinking in terms of their best interests. Basically, in most cases where I go to war with another empire, the only way for the war to stop is for me to pay the computer off. Which I am not likely to do when I am winning. Most of the time, the computer fights to the death, even though they get continually weaker as you capture their cities/planets. I usually accept the rare instances where the computer sues for peace.
I have that much bitching to do because I've played the game a lot. Why, though? Why deal with that?
Because I LOVE the space game. Quite simply put, I haven't found a space empire game I've liked as much as the Space stage in Spore since Master of Orion II (ok, granted, there aren't any major titles to compete with it, but I've tried some Civ mods that tried to duplicate the setting). I don't necessary like the first part of the Space section, when you're struggling to get established, but I love the scale of it. I have one empire that has dozens of star systems under it's control. They are masters of their region, with fantastic riches and most of the best tech. But I still find it fun to go through and develop their planets, and occasionally even grab new ones.
Even exploring, which I don't do that much, is fun. I love seeing new planets (though it was a big disappointment to realize the game does repeat geographic patterns, it took me a damn long time to realize it was happening, which speaks well for the variety on offer), and I love watching them change due to terraforming.
Even the earlier stages have their moments. Watching your tribesman successfully best an Epic creature is great fun, especially if you can then turn it into a giant BBQ.
Also, there's still more to explore. Just the other day, I created an a race with an ideology that I hadn't previously known how to create (sure, I could ask the internet if I was really curious), and the fun of that discovery keeps me going, and trying new and different things.
So, Spore puts me in the unusual position of liking a game, but not being about to recommend it. Play only if you're as crazy as I am.