IMO the voice acting in the full game (haven't played the demo) is outstanding once you get used to the characters. Especially Auron, Wakka, and Rikku. All of those actors did a marvelous job with thier characters, especially considering that they had to compensate for the still-present overexaggerated gestures of the old FF games and synchronizing thier speech and thier tone to suit the game's animations. Alot of people forget that unlike CG movies and things that are made in america, the visuals were not conformed around the (american) actors. Even in Anime, wordfit can be used to adjust the lip synchronization. These guys had to wrap themselves around the characters and did a great job doing it.
Some of the smaller "bit" parts could have used better voices though.. in a few of them it seems like they yanked the janitor out of the hallway and said "Here.. lower your voice and say this line." But that's a graphical problem too. The emotion on the main character's faces are very well done, but everybody else has basically an FFVIII face with hinged lips. By this I mean that they only move up and down, up and down.
The story for FFX is also very, very well written. It's very depressing at times, and can border on being macabre (I don't know if I used the context right, here.. *shrugs*) at times.. but it is still a masterpiece, especially if you're attentive to details. A guy on the FFX Neoseeker forum went through the game three times and talked to every single person before he could wholly understand the game's plot. Some might say that's too complex but just think of how much effort it took to write all of that and make it properly fit together. (People have different dialouge the first time you meet them, and new dialouge appears after each of two events before the end of the game. I'm not gonna spoil it for people who haven't played, though.)
I still hope there's not a sequel, though. It would kind of spoil the story. (I even think that the FFX international ending is kind of weak, given that it was tweaked to be more of an upside end.. and it leaves the story even more open than the origional finale did..)