I maxed every1's grid... Got every1's ultimate weapon... owned Penance, Nemesis, all Dark Aeons WITHOUT Zanmato.
Here is my conclusion...
Every char has got its use .. However I find Lulu the most worthless overall vsing Super Bosses
Wakka = Attack Reels (12x99999 against Nemesis 12x30000ish against penance and Dark Aeons)
Auron = Banishing Blade (full break to super bosses)
Rikku = VERY GOOD supporting char
Tidus = Blitz Ace/Slice Dice (8x30000ish 1x99999 or 6x99999)
Yuna= summoning Aeons to block
Kimahri = only Mighty Guard is useful...
Lulu = nothing lol as all super bosses immune to magic
IMO: Best all out attack is Tidus/Wakka/Auron
Yuna is good to block off strong attacks by summoning Aeons
Rikku is good at supporting
http://www.youtube.com/watch?v=tUxUU6BzBUAsample video