It does get good at the climax of the base game, A Realm Reborn, when Rauban loses control. That's what hooked me. There are other great moments and plots along the way through the whole game . But for the overall best time, it's the 3rd expansion, Shadow Bringers, that's a masterpiece all the way through. That's some of the best Final Fantasy I've ever played from Squaresoft or SquareEnix.
Dawntrail, I can't recommend. I can't get into it.