Hmmm, weird. I got interested pretty early on in the game when Tidus got zapped into Spira and tried to find his way there. Also when Yuna went through her journey, but still early on, I thought something wasn't right with the whole premise of the thing, so I got vested into wanting to know what the caveat was, and from then on, I was hooked. Well, actually I was hooked by Luca, but hey.
Yeah, the game has its flaws. Tidus can be a bit of a douche at times, but dammit, I admire that he had the balls to stand up. I got vested pretty on in Tidus and Yuna, and was hooked through the conclusion. Also I liked the character development that everyone went through.
It's one of my most cherished games, and I've played through and beaten it 5 times through the years.