After maxing the entire crystarium on all characters, I noticed that some of the defaults were pretty bad and some of the side roles were pretty good. Fang as a Syn is awesome, because she's the only one to get -ra level of all the useful spells. I use her as a Syn when I need to kill Long Guis. Light as a Sab works really well, too, as well as a Syn. Making Fang a Rav is amazing, especially when I got her strength as high as it would go. Then, when I switch her back to Com, her attack hits like a truck. Sazh would make a good Sab, if he had more Strength/Magic. He has the lowest in both out of all characters... Then making Hope and Light Sens works well because they are the only ones who learn Evade, which is one of the most useful spells in the game. Primary roles should be maxed first, though, because they take far less CP to max, and provide 10x the stats. After that, though, start branching into the trees that they get really good spells for.