I'm only about 5 hours into this game, and already it ranks on par with The Legend of Zelda: Ocarina of Time. Whether or not it actually surpasses it, I will know when I've completed the game.
As it is, this is game is nothing short of amazing. People were disappointed when they first saw Wind Waker, myself included, but when it came out, it was a great experience - the cartoon graphics worked, and the story linked back to previous LoZ adventures.
Now that Twilight Princess is out, you can really see that Wind Waker was the testing grounds for this game. One thing that stood out for me in Wind Waker was the expressions on the characters, like the movement of eyes. This has been translated over to Twilight Princess (it does run on a modified version of the Wind Waker graphics engine). People complain about the lack of voice in this Zelda game, and they have a point. But to me the expressions of the characters make up for it. They say more than words ever could. The developers have spent a lot of time on the individual characters, and watching the faces of Link, Ilia, Colin, Midna etc etc is just mesmerizing.
The graphics in this game are excellent for a Gamecube title (but still not taking full advantage of the Wii hardware), obviously not on par with Xbox 360/PS3 games, but they do the job for me, and still feature impressive effects (love the shadows of clouds rolling over the ground)
The music is excellent - it is original, yet harkens back to Ocarina of Time. To Zelda fans - listen and you'll even recognise music from A Link to the Past. Sound effects service the game very well.
Overall - Legend of Zelda fans will love this game, as I surely do. But you do not need to be a fan to get into it - anyone can, but be prepared for a few challenges, as the game definitely is not a walk in the park.
P.S. Gamespot - This game deserves more than 8.8. 'Nuff said.