I've played the 'full demo' and here are my opinions:
Cell Phase : Actually kind of fun, because I liked fl0w. A cute minigame that occupies you for about ten minutes. I played as a carnivore, while my friend who played as a herbivore said it was mind-numbingly boring. YMMV.
Creature Phase: This is my favorite part of the game, and honestly, the only thing which I think was done right. It's challenging and fun, although the stats from body parts work oddly, as has been said elsewhere in the thread. (perhaps to mitigate the benefits of powergaming?) There are epic creatures here which really are epic-sized but you haven't got a snowball's chance in hell of killing them, at least without powerful allies (another epic? I don't know if you can convert them, but you can definitely convert rogues)
On the other hand it's all over far too quickly and there aren't enough body parts (this is probably EA's animalistic lust for expansion packs talking). There are essentially four different 'styles' : weird alien things (eyestalks, spindly hands, pincers, suckers) bears (claws, sharp teeth, paws) and...two other styles that I can't remember, but there are basically four levels of each style for each limb...
Having played as a predatory carnivore, YMMV.
Tribal Phase: This plays basically like a bad RTS, complete with the bad pathfinding. You have three types of units: axemen (good against infantry, bad against buildings) firestarters (bad against infantry, good against buildings) and spearmen (bad against both, but ranged). Firestarters are completely pointless as there are no creatures who fight back, and spearmen aren't smart enough to 'skirmish' and so get killed pretty quickly, so you pretty much have to mass axeman and just rush at enemy camps. And you will win. It takes forever, but it's not difficult.
There's an option to 'impress' other tribes into your own but I've not used that.
Civilization Phase: See above, but with cities. If your planet is all on one continent I guess you could just mass tanks and conquer the planet. You can design your own units and the AI tends to have 'balanced' units which are equally bad at everything, so you can pretty much steamroll over everyone with high-attack units.
This takes forever and is really boring. There are some special abilities including ICBMs which, when I used, pretty much won the game for me as there were only three rival cities left which were immediately destroyed.
Space Phase: I had high hopes for this one. I expected that every other phase would be a footnote compared to the space phase. How wrong I was.
This is essentially a really bad RPG...in spaaaace! You go from planet to planet, doing fetch quests or 'killing wildlife' quests for 'Sporebucks'. There are also pirates, who seem to be there PURELY because someone in EA thought 'It's not sci-fi without space pirates!' Seriously, there is no explanation for why they are there, no reason for them trying to kill you (it's not like you ever lose anything by dying, so what could they gain?) and they don't even have a representative species; they're just 'space pirates'.
The terraforming/sculpting aspect of the space phase is there, but it has been hung, drawn, quartered and gutted and then its foul corpse has been brought back to life by some kind of evil necromancy. The tools on offer are various environmental changers (temperature and 'atmosphere', which seems to be always breathable by anything and changing it basically means adding or taking it away) which, annoyingly, can only be bought one at a time, and you need something like 2-3 environmental changers to turn an inhabitable planet uninhabitable and vice versa. You can also 'paint' the skies or the ground in different solid colours (no shades here, but coming soon, Spore: Terraforming Paint Stuff! Only $19.99.) and change the ground into different patterns, like...chocolate, and tentacles, and 'cute canyons' and 'crystal chasms' (none of the crystal things I unlocked worked for me
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The interactions with species are either really annoying or really simple. With pre-warp species in the tribal phase you can abduct them, kill them all or leave crop circles (the purpose of which I have yet to figure out). With civilization species you can do the same, except you can also scan their buildings and learn about their designs, and it takes far, far, FAR longer to kill them. (It took me about 30 minutes to wipe out a map of sentient civs with the medium-strength weapons) Also, alliances and trade routes require CONSTANT attention. If you aren't continually supplicating each of your allied races they will throw a fit and stop cooperating with you until you give them more cash.
I guess the point of this is eventual galactic conquest/hegemony like in the other phases, but you also get 'badges' for doing things, like you will get 'Missionista' badges for doing lots of quests and you will get 'Gopher' badges for doing lots of fetch quests, etcetera. It's basically all grinding and none of the fun world-smashing where you are almost omnipotent, as the previews seemed to imply...
All in all I think Spore is a decent enough collection of minigames but as a game that took more than three years to make and also required EA to remove any limitations on budgetting for Maxis, I have to wonder what the hell they were messing about with unless they were busy making the sixteen expansion packs in advance. (If you think I'm exaggerating, I'm not; Sims 2 had about sixteen)