I'm not the biggest hand-held game fan in the world, but DBZ: The Legacy of Goku 2 is worth a try.
Gameplay wise, it plays similar to an RPG. Fight creatures along the way while leveling up your strength, how long your ki lasts, health, etc in order to have a better chance at progressing in the game.
Storywise, it follows the events surrounding Trunks' debut all the way to the final battle with Cell, so you have a lot of work to do in order to beat the game. You take control of both Goku, Gohan, Vegeta, Piccolo and Mr. Satan (unlocked after beating the game for the 1st time while leveling everyone except Goku all the way to Level 50).
Combat mechanism is pretty simple and the A.I. is as straightforward as you can possibly get, unless you're crazy enough to be ganged-up by several enemies at one time with a low level.
All in all, DBZ: The Legacy of Goku 2 is a REALLY good game for the Game Boy Advanced, but it might not bode well for everyone's taste. If you're a really big DBZ fan, it deserves at least a tryout.