I haven't played it. My wife has it, and my daughter has played it on her computer- she wishes it was possible to stay in the "creature" phase rather than going on to the later tribal/civilization stages.
My impression is that Spore was (heck, is) a great idea that just didn't quite play out to the scale or scope that many people were hoping for- that at its heart, the game play remained kind of shallow. I felt my first unease in an early demo where one of the creatures picked up a spear. At the time, I remember thinking, "Why does it have to be a spear? Wouldn't it be far more interesting to see what sort of tools, vehicles, or weapons a creature that didn't have a hominid physiology might devise?" And to me, that's about the size of it: for all the Mister Potato Head monkeying around with bits and pieces- which is certainly fine, and kind of fun- you're still running the same pre-designed laps.