i've been using lightning, hope, and snow a lot, but i'm about to switch lightning out for sazh. i'm getting sick of having her get hammered thanks to her relatively low HP. i know she's my most versatile character, but she gets pounded with relative ease by most bigger enemies.
snow's awesome with the power circle as both a sentinal and a commando. i'm in chapter 12, and he's level four in both - and he's kicking ass in both. he'll easily hit for 8000 per attack, per turn, against a non-staggered opponent, using the base level of the power circle =) he's also a way better sentinal than fang thanks to his superior HP.
i'm digging the syn-syn-sen at the start of bigger battles. hope's buffs work perfectly with sazh, and you can get all seven major buffs (veil, protect, shell, haste, vigilancy, bravery, and faith) on all characters in the time it takes to normally have two or three turns.
the main issue with sazh as a leader is that you don't have that emergency paradigm of combat clinic (med-med-sen). if you use enough synergy paradigms, though (i always use one after staggering an enemy, when there's no chain to maintain), it balances out usually.