He is my favorite!
I liked his story, yet it was so sad. He is silent, but he has the reason for it.
He can be really powerful, and USEFUL if you care about him through the game.
The most annoying thing I always read about: "He is not useful!" What is this crap? Since everybody can learn everything in FFX, it's up to you. If you don't care about him, of course he won't be useful...
But if you do, you won't regret it. Nova rocks.