Mario and Luigi: SuperStar Saga is a great game. One of the best on GBA. It mixes excellent platforming, puzzles, mini-games, and RPG style battles in a seemlesly. However there are a few things keeping SuperStar saga from reaching superstar status.
First off the game is pretty short, especially for an RPG type game (first playtime of about 16 hours). And there is pretty much zero reason to replay it. Also The world that you explore is not very big. And you have to freqeuntly move around the central area, so it gets a little repetative.
But these are minor complaints. They only keep this game from being incredible, it is still great.
The thing that makes this game so funto play is the crazy variety of things you will be doing. While moving about the world with the brothers you control them both, and they both have "bros" abilities that let them work together to access places you wouldn't normally be able to. This makes for a fun and fresh platforming experience.
In about every new area you come across there is also some mini game that you usually must complete to procede. But they are short, fun little deversions from the main game.
The battle system is alot like Super Mario RPG, for those of you who played it (if you haven't you really should!). If not any turn based RPG will do as an example. But the thing that makes Mario and Luigi different is that you are constantly timing button presses in the battles. You must time a press during your normal attack to do the most damage. You time a press to avoid enemy attacks while on defence. And during your special "bros attacks" (the games special abilities) you must time severall presses. This is where the game is most like Mario RPG, but the timing presses in Mario and Luigi are much easier to time. However they are much more important. You need to hit all the time presses for your attacks to do any real damage. And on defence a correct press completely dodges attacks, this is very important because you have very little HP.
Overall the only real complaint I have is that is was to short. But better love the game and be sad it's over, then be waiting for it to end.