Without a doubt one of the best games I ever played for the Nintendo DS. If you enjoyed Superstar Saga and Partners in Time, then you already have this game, if you aren't familiar with the whole Mario & Luigi RPGs this one is a good place to start.
In Mario & Luigi RPG titles you take control of both Mario and Luigi, one button controls the actions over Mario while the other button serves to Luigi, which usually follows Mario. As such, expect to jump to platforms with both brothers at the same time, but is not the selling point. This control and display of characters allows for numerous puzzles, for example when Mario and Luigi get a Hammer, Luigi can hit Mario so that Mario becomes smaller, allowing him to go previous unreached places. Mario and Luigi also learn techniques that allow them to dig and fly for a short period of time, so you can expect a lot of creative platforms.
The combat is similar for all RPGs, however Mario and Luigi can defend or even counterattack when an enemy is about to hit you. With practice, you will learn how to dodge all major attacks. The timing also plays a great roll in combat, all Luigi and Mario attacks allows for combos if you hit the button just in the right time.
The game takes huge advantages of the DS hardware, the level of creativity is something you would expect from a Zelda title. Before this one I played Zelda:Spirit Tracks and I have a lot more fun with this game. And don't get me wrong, I'm a huge, HUGE, Zelda fan, but my point here is to show how awesome this game is.
The big star is of course Bowser. I can't tell you how much I Laughed with the character: his behavior, laugh, conversations with his minions, it's just fun to watch him.
If aren't convinced by popular review sites (such as GSpot, IGN, GameTrailers and such) see what Ben "Yahtzee" Croshaw from Zero Punctuation as to said about this game ;) Buy this now.