@Blue: considering each playthrough is completely unique because of who you play with, I think that it's worth the price. Hell, it's about the price of a movie, the same length, and better than most stuff out now.
@Ginger: No villain. No real conflict, even, aside from some snake-beast who take away some of your power, but there's no way to die. A cynic might call the game a sight-seeing tour, but there's a lot more to it than that. There's a deep sense of awe permeating through the whole thing; you're an incredibly small character in an incredibly huge world, and for the most part there's nothing else around aside from a few small magical animals. It's absolutely breathtaking, and as you explore through the ruins of a dead society you really get a feeling of morality, and temporariness.
The really good thing about it is playing with another person, though. When you play with an internet connection you can randomly happen across another player (only ever one at a time) who you can take the journey with. It's absolutely beautiful the sense of comradely you can form. A lot of the levels in the game facilitate playfulness, like sections where you just slide through mountains of sand, weaving in and out of dead buildings with each other, or flying with each other and keeping the other afloat - you have a finite amount of change for flying, but you can recharge your friends power by singing or being close. So if you stick together, you can fly infinitely. It's just absolutely beautiful how can stumble across some random person and become friends with them, working through all these trials together. You actually feel bad when you see them fall, and you want to stay close to them the further into the game you get.
it's essentially man vs. wild in a game format, but the benefit comes from that small fleeting contact. Even beyond that, the game world is
incredibly rich and beautiful, and there's a deep sense of mysticism and awe. I literally cannot sing this games praises enough. It's so amazingly well designed that it's almost heartbreaking for me.