If you're not living under a rock, you already know most of what I'm going to say. Let me put this very simply: I love this game. It has some flaws (like DRM, which only lets you install 3 times until you have to phone up the company and beg them for more) but it's still amazing. It's just incredibly fun to play, and the humor is great.
Plays like: FlOw
The good: You get to swim around in a primordial pool teeming with life. It's very beautiful.
The bad: Most of that life is MUCH bigger than you and is determined to eat you. For every creature you can beat up on, there are 5 that could easily own up your zooplankton behind. Your only option is to get on land ASAP.
The good: This is my favorite phase, hands down. You start out as a single individual of your species on a vast, unexplored continent. You can periodically come back to the nest for healing and mating. As you explore, you encounter the nests of other species, and you can then choose to mercilessly exterminate them or complete a bunch of emasculating and somewhat tedious befriending games with them. I chose the exterminate path and had a blast leading my warband and carving swaths of destruction with them.
The bad: It takes a long time to unlock all the upgrades, tier 1 walking is painfully slow (so upgrade fast!), and sometimes other species' nests are hard to get to when they're on hills, anything more than a 10 degree incline is apparently beyond your species' ability.
The good: As a fledgling tribe, you get a small camp to build up, and a handful of troops to command. Similar to the creature stage, you can either exterminate or befriend your rivals with instruments. If you befriend another tribe they'll periodically send over some food, and the spectacle is adorable (plus the Mariachi music is hilarious! :D)
You really have to be on your guard on this one, as at least one of the 4 other tribes will attack you aggressively. Fortunately, the combat is pretty easy.
The bad: At max pop, there are sometimes pathing issues. I made the mistake of telling my army to eat up all at once and a couple of them (including the chieftain) got stuck somehow and almost starved. And if you made a non-humanoid creature, it's quite hard to dress them up in tribal gear. Putting a grass skirt on a centipede-like monstrosity is quite the challenge.
Plays like: a scaled-down version of Command and Conquer
The good: As a fledgling city-state, you have to conquer/purchase/convert enemy cities to win. If you go military, it's extremely easy, you can just tank-spam the entire continent to death fairly quickly, then build airplanes to quickly demolish any cities that aren't on your starting continent. Converting isn't much more difficult, but it seems to take a little longer (but makes up for it with the menacing hologram). I haven't figured out purchasing yet, so I don't know about that.
The bad: It's a little too easy, especially considering how difficult some of the other phases are. (see below)
Plays like: A space version of Civ 4 set to deity difficulty.
The Good: Here it is, the big enchilada. The endgame. The sandbox. And for the most part, it's extremely well done for something this immense in scale. You get to command whole planets and fight wars on a galactic scale, and battling other spaceships on the planets is incredibly fun, as well as the struggle to unlock the huge array of badges and powers. But since it's 90+% of your gametime, its flaws are extremely noticeable and persistent.
The Bad: It's not really much of sandbox when you keep getting hit up with missions all the time. And even worse, many of them are timed missions. Whether it's an ecological disaster (with a 3-minute timer), one of the frequent pirate raids, a Grox attack, one of your allies under attack, etc, you are guaranteed to be nearly ceaselessly occupied with crisis after crisis, with little time to explore or even figure out how to play this phase.
The enemy empires are extremely aggressive, right off the bat. I had several nearby empires demand tribute constantly (with ever-increasing demands). Eventually, they gave up on the charade and attacked me. I also had empires that I hadn't even heard of yet declare jihad on my species (the religion aspect of the game really is pretty annoying, with a surprisingly large percentage of alien races acting like a cross between the Ur-Quan and a drunk Mel Gibson). Almost immediately after one attack was fended off, there would be another one, and they had huge empires in comparison to mine.
Terraforming: unless you have all the cool terraforming powers, it's extremely long, hard, and expensive to make any T3 colonies. The learning curve is pretty rough, so I had a tough time stabilizing the ecosystems with the right plants and creatures.
Money: there's never enough of it. All you get is spice, but trading it only gets you a few thousand. Meanwhile, you're practically guaranteed to have ginormous rebuilding bills for all your constantly-attacked planets in the upwards of hundreds of thousands. You get broke and you stay broke.
Lastly, and this is a minor criticism compared to the other stuff: the ship's tracker isn't very good. Sure, you get an audible ping and a fairly ambiguous arrow every few seconds, but it's not especially helpful. It'd be nice to get a colored trail to the object or at least get your target highlighted on the screen, but that doesn't happen, so it's a squint-fest hunting for the barely visible critter you desperately need for your colony's biosphere (all while being bombarded with missions to defend your empire or timed ecological disaster missions).
So, unless you're a masochist or have access to some excellent cheat codes (I bit the bullet and used the money cheat relentlessly), you might want to stay away from the space phase until it gets patched up to something playable.
Final Grade: A-
Despite all my complaining, I really did love this game to pieces. For the immensity of what it tries to do, it does most of them extremely well. And the deluge of user-created content is great. I love seeing my old creations roaming the myriad worlds of the Spore universe.