Mario and Luigi was the GBA's closest thing to Paper Mario or Super Mario RPG.
Though they are close to their Mario RPG cousins, Superstar saga had it's own unique battle system. I personally enjoyed the battle system out of any of the RPG games. The combo attack with the two brothers gave it variety, and tested your skills with the more advanced moves.
I also loved the crazy ass story about Bean Bean Land. The quest was long and epic, with a juggernaut of a final boss to tackle on.