Quistis is one of those characters that I almost always have on my team (which is usually Squall, Zell, and Quistis). Her unique abilities make her a very good member to have in your team.
First off, her attack is usually stronger than that of Selphie or Rinoa. I know Rinoa has a higher attack stat range than Quistis does, but on average, Quistis' attacks tend to do more damage. Though Quistis' attack is a heck of a lot weaker than that of Squall or Zell, her attack isn't the primary reason to have her in your team.
The best way to utilize Quistis' abilities is to set her as the healer // status remover // status inflictor // magic caster. Her defense and speed are really good and her limit break proves very, VERY useful in most battles. Her best (in my opinion) limit breaks are White Wind (which heals the amount of damage you have taken to everyone in your group), Mighty Guard (casts protect, barrier, and has a chance of casting float and aura on all your team), Bad Breath (this sucker casts ALL status problems on your enemies), Degenerator (instantly kills one enemy with a high success rate *doesn't work on bosses of course*), and of course, none other than her strongest limit break, Shockwave Pulsar.
Most of the limit breaks that I just listed are of the defense kind meaning that Quistis is great for protecting your party with her strong magic spells that don't cost MP and can be reused anytime just by casting aura on her (which would automatically be on if you cast Mighty Guard). Her limit breaks are also reliable unlike most all the other characters where you have a random chance of getting a specific limit break, hers are constant and you can choose which ones you want which (in my mind) makes her far more valuable than Rinoa or Selphie. Both of which have a random chance of getting the limit break you want.
During battle it is best to keep her as a primary healer and a status inflictor. During boss battles keep her casting White Wind to cure all damage you might take and cast Mighty Guard which will heavily protect your party at basically no cost of yourself. Whenever you get the chance, have her cast Shockwave Pulsar (which can cause 5 digit damage making her attacks far stronger than Rinoa's or Selphie's) and your battle will basically be won. Another cheap trick is using Quistis' Degenerator in battle against a pesky foe such as a Malboro or Ruby Dragon. It has a high success rate of knocking that baddie dead in one hit.
As for what GFs I usually have Junctioned to her, it's usually Shiva, Siren, Leviathan, and Eden. ^_^ Cool team that gives her unlimited cool abilities. Sometimes I even throw in Diablos in there to give her extra cool abilities like mug and devour.