This is the third Mario-RPG, a series who's latest game before this was the fantastic Paper Mario on the N64. While Mario & Luigi doesn't quite match that game, it is still very, very good.
Graphically, it's a beauty, colorful, sharp and with great-looking enemies and bosses and without any of the little slowdown that Golden Sun at times suffered of.
Sound-wise, the music is really good, with lots of pleasing tunes, strong sound-effects (by GBA-standards of course) and funny speech from Mario and Luigi.
The story is surprisingly strong, as weird as weird can be, but with surprising plot-twists and featuring some memorable moments. Quite amusing as well.
The gameplay is what matters most, though, and it delivers here too, with a well-designed world packed with puzzles, mini-games, battles, not to mention various of challenges. Maybe not as advanced as Paper Mario, but still big enough to last a decent while.