Yuna, Rikku, Payne.  That's it.  You'll also see other characters from the original FF10, but they won't be playable.
If the story is as good as FF10's story was, FF10-2 will definitely be worth playing.  My only concern is the battle system.  If it's like Kingdom Hearts non-turn system, I'll probably hate it.  If it's the traditional turn-based system like all the other Final Fantasies, I'll love it!
Have you guys seen the trailer?  It looks awesome!