I'm loving the game!
Each part is quite quick and fairly easy. I mean, come on, you can't have Command & Conquer, Civilization, and Galactic Civilization all in one box plus a brand new genre of game for the creature phase. The game would cost $200 and it would take you weeks to finish!
Spore's big claim to fame is the development of an entire species - and then some. In that aspect, it's a lot like developing a character in an RPG. Only, unlike the vast majority of single player RPGs I've played, I've got a real strong desire to play again and again. Depending on how you run through the game, you get different powers in the next phases.
Space is a very special phase. It's MUCH longer than all the other phases combined. You could make an argument that those other phases are, in reality, a glorified character creation scheme that determine your class when you enter the space phase (I think there are 11 of them). There's a heck of a lot more to do in space, too. Set up trade routes, do missions for aliens, terraform planets, attack aliens, spice trading, setting up cities on your new planets, gathering artifacts, and so on.
It is NOT a multi-player game in the normal sense. However, your creatures (and vehicles and planets and space ships and....) get uploaded to the massive Sporepedia and automatically downloaded by others. People you you mark as a 'buddy' will get high priority on your downloads plus there are 'sporecasts' if you are interested in some sort of specialty. All the fears/laughs about "sporn" seem to be unfounded, too. When I first started the game on the 7th I found one critter downloaded that was just a tad raunchy. I banned it and haven't seen anything bad since then. And it's actually quite fun to ally with, domesticate, and/or wipe out your friend's creations.
My big beef with the game is the difficulty level. At medium difficulty the cell phase is easy, creature phase is medium, tribal is easy, civ is easy, and space starts easy but turns into death incarnate. It's really a major jump - so major that I suspect a last minute bug cropped in. I tried running two games on medium difficulty and both times I flew through the first phases only to get clobbered in space. (And actually, MOST of the space stuff isn't too hard, either - it's just the wars where enemies send fleet after fleet after fleet at you that are killer.) On easy difficulty, it's no problem at all.
I found a little easter egg, too. Every once in awhile, when talking with one of your colonies, the theme music from M.U.L.E. will play. Took me a minute to figure out where that song came from and I about fell out of my chair when I got it!