I'm not gonna argue with the above. The game never truly opens up. It becomes a round, circular corridor that's less pinned in rather than more open. After all the narrow corridors, I suppose you could be forgiven for seeing Gran Pulse as a huge open world, and the snaking areas as divergence points. The gameplay becomes more open via the Cieth Stones and Titan's Trials, but the exploration aspect simply becomes less pinned in.
Of course, it's more down to a method of perception than anything else. Final Fantasy XII offered several divergence points, and multiple snaking ways of getting from one area to another. There were around three or four ways to enter each new location, and the Great Crystal in particular was incredibly diverse and complex, something which made many players happy.
Final Fantasy X on the other hand is rather linear in itself, and it's a game close to the classics in terms of popularity. You were just led down a linear path that disguised itself as something more. The Mi'hen and Djose Highroads and the Omega Ruins were big offenders here, and the Calm Lands is comparable to Gran Pulse in its just being a less pinned in area.
Frankly this is down to a lack of world maps in newer games, and newer JRPGs in general. Whether they're having trouble programming this into the new generation of consoles or whether it's a case of laziness in keeping the incredibly good looking graphics running across an entire multifaceted world map, it has a big part to play in the recent constriction of exploration and diverse paths in the Final Fantasy series and RPGs as a whole. Final Fantasy XII came close to recapturing them but for whatever reason, they chose not to pursue that in Final Fantasy XIII, perhaps because Final Fantasy XII as a whole wasn't incredibly successful as a singular completed product.
I've gotten off topic a bit, but there it is.
The characters themselves I found compelling enough but if you've finished Chapter Eight and still feel nothing for the characters and the story in general, then you won't find satisfaction in continued play. Chapters Seven and Eight and to an extent Nine are incredibly climactic story wise, while Twelve is more climactic gameplay wise.
I will say though, the shift in tone is rather...disconcerting. The first eight chapters have their lighthearted moments, but at their core is an aspect of cynicism and the impending danger towards the main characters in a world that despises them.
Come nine, when for the first time you have all six party members, well things suddenly shift towards optimism so quickly my neck hurt from the whiplash. Chapter Nine, the first half of Chapters 10, 11, 12 and most of 13 are just coated in so much sugar I was diabetic by the end. The cynicism returns here and there, such as with Cid's fight, Hecatonacheir, Oerba, Yaag's death and the ending, but the feeling of a sudden shift in ideology lingered still. It was and remains my biggest complaint about the game.