The single best game I have ever played, and my favourite of all time on any console.
The physics all work very well, for example when on the bird's wings (this was one of my favourite fights) you are slowed down as expected by the wind battering at you. But you don't slow down too much or too little. It feels just right. The game controls amazingly with all the Colossi.
The game also has puzzles, and some of these are integrated into the boss fights. I don't really want to spoil anything, but for example, in one fight you need to climb up a Colossus' arm to get to the weak spot, but a stone "bracelet" blocks your way. You have to do a certain thing to break this "bracelet" before you can actually climb up its arm. In another, you have to hide in a cave until the Colossus peeks into it, then run and jump onto its beard to climb. All of the puzzles have certain hints, and they can be hard, but they are usually just right. The overall difficulty of the game is fairly high, so not for inexperienced players.
To move between Colossi, you have a horse named Argo, who will always come when called.
The game is exceptional, and ever since I played it on my PS2 which I sadly no longer own, no game has been able to come close to matching it. I am anxiously waiting for the HD PS3 releases of ICO and SOTC.
If you own a PS2, buy this. It is a truly unforgettable game.