Ok, having completed all the major objectives of the game, I'm going to throw in my final $0.02 on the game itself.
Spore had a lot of potential, the game was in development for so damned long it made a few vaporware lists, and most of us who were following it had very high expectations for the game. Knowing this, I tried to go into the game with as few prejudices as possible so as to not be disappointed by features that didn't make it in that were shown in the previews. Ignoring those, of which there are many, I still find that I'm seriously disappointed by the game as a whole.
I feel this is best addressed phase by phase.
This phase is interesting, but overall is completely pointless to the progression of the game. The only impact this phase has is what special abilities you get in later phases. None of the creature parts can be used in later phases to any great effect, and the only realistic way to become an omnivore at this point in the game (as far as the little "alignment graph" at the end of each phase is concerned, anyway), is to immediately stick two mouths on your creature. Otherwise, by the time you get the one omnivore mouth towards the end of the phase, you've already eaten far too much meat or vegetables to change it back.
The phase itself it also fairly boring. Cells cannot be shared on the sporepedia (as far as I can tell, anyway) so you are always looking at the same 30 or so cells that Maxis put into the game. By the third or fourth time you play through the cell phase, you've seen all of them, and that's not the first impression you want people getting in a game that's supposed to be ever-changing and fluid.
The creature phase:
This phase is arguably the most fun and polished phase of the game. The creature editor, which is the one most people playing the game are most familiar with due to the early release of the creature editor, is quite a bit of fun. The creatures you run into however, are pretty bland, and basically boil down to "This creature has high social/attack, so I need to kill/dance with it". Getting mouths other than the one you got in the cell phase is impossible. If you were a carnivore in the cell phase, you cannot be an omnivore or herbivore. This seems to severely limit creativity in this phase based on an (unviewable during phases) ranking that you have little control over due to the availability of mouths in the cell phase.
That aside, the changes you can make to your animal are too extreme. You can literally, in the course of one generation, completely and totally change your creature without any regard to anything except mouths. For a game that seems to want to be about evolution at this point, this couldn't be wilder. In a single generation no species can go from 6 legged flying insect to a bipedial mammal. The distinction of mammal, reptile, insect, and others, is also notably lacking from the game. Everything lays eggs, and everything appears to be a hermaphrodite.
The tribal phase:
This is where things begin to feel a little rushed. There's very little strategy to the tribal phase and to be honest, only one strategy. Trying to fight the other tribes is a losing proposition as they have equal or near-equal numbers and usually equal to superior weaponry. The only way to win a fight against them is to exploit the AI's tendency to reproduce slowly. You'll have completely rebuilt your tribe while the enemy tribe is still messing around with 3 members.
On the other hand, it's laughably easy to simply sing and dance your way into winning this phase. Even if a tribe hates you so much that they will kill you on sight, a single very small gift of food will put them back into neutral, and your chieftan and a few helpers can wander over and make friends with them.
This phase is very short, usually only lasting about 10 minutes, and then you're off to the next phase.
The civilization phase:
This phase suffers from many of the same problems as the tribal phase. War is almost pointless (religious war even more so, as the vehicles are easily countered by entertainment buildings) and trading makes it too easy to win. By the time you've traded enough to take over one or two cities, taking over the others takes mere minutes. Simply build a fast economic aircraft, make a lot of them, and set them to trading. Within minutes the city will be up for grabs and you've got more than enough money to purchase it. Completely pointless phase partially due to how few cities are required to advance, the inability to build more cities (requiring you to take over other cities), and the overall small size of all the planets.
This phase also has the dubious distinction of being the one to throw SEVEN new editors at you in short order, resulting in creative burnout by the time that last editor rolls around.
The space phase:
Ah finally, a spacecraft, and the spacecraft editor. Unfortunately, your spacecraft might as well be a featureless disk for all the effect the various modifications and pieces have. Every bit of the spaceship editor is cosmetic, none of the parts do anything to gameplay. While this is similar to the building editors in that respect, it is a major departure from the vehicle editors, which the spaceship editor more closely resembles.
The space phase itself starts out rather swiftly. No sooner than you learn to fly your spacecraft, they give you an interstellar drive and you're off to deal with alien species.
Much like the tribal and civilization phases, war is generally not the answer to your problems in the space phase, and why would you bother? Even the most violent of races (with one notable exception, see below) can be nearly instantly bribed into total complacency. Unlike the civilization phase however, the process of buying out star systems is extremely frustrating. You can only have 3 trade routes open at once, regardless of if they are with the same empire or multiple empires. The trade routes themselves don't make you any money, and the cost of the systems is ridiculously high. Should you underbid, and there seems to be no actual rhyme or reason to what a species will ask for a system, they will get angry at you and you have to wait for the trade meter to fill up again, which can take quite some time.
Getting money in this phase is tedious and repetitive. The only way to get money is either boring kill/fetch/scan missions which will seem distressingly familiar to anyone who has played Eve-Online, or running spice back and forth. Maxis tried to make it seem that there's some complicated process by which different species decide how much spice is worth, however it basically boils down to "Planets pay more for whatever they don't produce, +/- a random percentage". Planets that minutes before, regardless of what was sold to them, were paying princely prices for a type of spice, are now paying a mere pittance, with no discernible reason.
Another major annoyance with this phase is having to constantly babysit your planets. No matter what you do, you will always have one of your planets, or an allied planet, have something happen to it. You then have to fly back across the galaxy from wherever you are to deal with it, or watch your planets slowly and methodically get destroyed, getting you that wonderfully taunting "Careless Parent" achievement.
Then of course, I must address the one concrete "Goal" of the space phase, which is to find what is at the center of the galaxy. To get there, you must pass through and (unless you make a herculean effort) get into a war with, a massively aggressive species that controls some 500 stars surrounding the center of the galaxy. They have better ships than anything else in the game, hit harder than anything else in the game, and wonderfully enough that gigantic interstellar drive 5 you picked up is rendered useless as soon as you get anywhere near the galactic core. Your jump radius is instead limited to the smallest possible circle, requiring you to find a path while being pummeled by a half dozen enemy ships, with no support or resupply facilities anywhere nearby, to get to the center of the galaxy. Of course, as soon as you annoy this species enough, they declare war on you, and you have to let them ravage your home worlds as it takes a good 10 to 15 minutes to get in or out of the area immediately surrounding the core.
You'd think, however, that having to push through (as there's no reasonable way to destroy them) an enemy of this sort, that is going to be attacking you from here on out, would reap an incredible reward. This is not the case. I am not going to give any spoilers here, but what is at the center of the galaxy barely merits the effort of getting there if this other species were not in place. Simply playing through the game to this point is all that this "reward" is worth. Instead, you get to initiate a permanent increase in the difficulty of this "sandbox" for a pointless thing that doesn't do anything you can't already do (And has limited uses, and doesn't appear to be rechargeable).
Now, there's a few things that don't fit into any phase that need addressing.
1: This game has many and various interface flaws that are inexcusable coming from a company like Maxis. No autosave? Non-remappable controls? The interface options for this game are a joke.
2: There are two very obvious editors missing. Firstly, a plant editor. You're currently stuck with only the plants that Maxis released with the game. Secondly, a hut editor for the tribal stage. I suspect these were either not ready at release time (which I highly doubt), or were removed so as to be released in an expansion pack later.
3: Graphics and sound wise, the game is average for 3 years ago, at best. I can see the need for World of Warcraft level graphics in a game that is going to have hundreds of different creatures on the screen at the same time. However, the game never has more than 15 or 20 creatures on the screen simultaneously, and therefore the bland graphics are less excusable. The game's sound is repetitive and annoying, and frequently has problems attempting to play more than two or three sounds at a time, despite my computer having more than adequate sound hardware, the process of flying in a straight line, playing the background noises, and making the radar "ping" noise seems to be too much for it to do at once.
4: The release of the game was a fiasco. I could download the game from the internet four days before it was released on shelves, and only my desire to have access to others content caused me to purchase the game. Even this was a pointless effort as despite there being over 10 million creations on the sporepedia at the time I wrote this, 90% of everything I see is still created by Maxis.
All in all, if I were in the business of rating games, which I'm not, I would give Spore a C+ at best. The latter half of the game feels rushed and poorly thought out, and too much of the game has a "Pending expansion pack" feel to it. I am sorely disappointed in both Will Wright and Maxis for this game, and do not expect to be bothering with their releases anytime soon. This game was in development far too long to be this mediocre.