Originally bespoken by James16:
Im jus really happy with the espers in general. I think they all look crazy, Ive only gotten Belias so far an his overdrive is jus jaw dropping an that makes me think jus how kick-ass all the other espers must be, an i like the fact that you have to beat them in order to get them.
Go to youtube, there's videos on there of alot of the espers, both using them and seeing their overdrives, and also fighting some of them. Also, you must not have played VIII, cuz you battled the summons in there before you could use them.

Oh and uh, I never managed to get the magus sisters, but if i recall correctly, they were the strongest Aeon in the game...