Zidane:
Pros - Helpful, kind, supportive, energetic, steal ability
Cons - Womanizing, hair, forced in to your party

Garnet:
Pros - Good white mage, good summons, haircut, I liked her development, serves as white mage for like half of the story
Cons - Trance, her disc 3 problems

Vivi:
Pros - Awesome, useful black magic, sweet, cute as a button, great character development, Trance
Cons - Uhhh

Steiner:
Pros - Goofy, fun, means well, lovable oaf, theme song
Cons - Kind of boring design wise

Freya:
Pros - Strong, intelligent, jump is useful
Cons - Loses relevance

Quina:
Pros - Goofy, fun, silly, cares about his friends, black magic
Cons - Looks ridiculous and leaves your party a lot

Eiko:
Pros - Strong for a six year-old, neat summons, excellent white magic, great trance
Cons - Can be annoying and bratty, bad outfit

Amarant:
Pros - Good skills, not too bad to use, fun to see him interact with others, especially Eiko
Cons - Looks ridiculous, only relevant for about 10 minutes twice in the game