The battle system is probably the most enjoyable in any RPG I've played, since it's real-time, which to me is much more fun than the point-click-and-watch style of many console RPGs. Each character has quite a large number of abilities overall, and two sets of abilities depending on which skill branch you lean towards. Combos factor pretty highly into the game as well, since you get experience bonuses and an increased chance of receiving items if you get high combos. Then there are the unison attacks where all four characters attack in succession, which look awesome while doing tons of damage (and there are even hidden skill combinations that result in extra crazy moves). The AI is quite good too, and you have lots of customization over their behaviour.
The game has huge replay value as well, since you can buy special bonuses (like EXP x 2) on successive plays and there are some sidequests that practically require you to complete them on successive plays since they are extremely hard on the first playthrough.