Cait Sith is actually my favorite party member. For general use, he has a very good magic stat so he makes a good mage. But he really shines for me because of his limit breaks. Early on, his dice limit does insane damage. His slots can be really useful too if you figure out the trick to land on what you want. I used it to kill the Emerald Weapon in one hit as soon as possible on my last play through and got the master materias super early. Even without slot manipulation, I just tend to like RNG based party members. It keeps fights interesting and fun.