Simply Amazing. Best Mario Game since Mario World.
The plot follows the plot of just about any Mario game, it's up to you to rescue the princess and Koopas are the culprits, naturally. The game uses an overhead map of the world just like previous 2D mario platformers. But all the main action is sidescrolling goodness.
In addition to the traditional moves and powerups you'll find some new ones as well that are fun and original. The cast consists of classic Mario characters, as would be expected. Lakitu, Hammer Bros., Bowser, they all make an appearance. It pays homage to the original Super Mario Bros. in certain ways such as flag pole jumping at the end of a level, etc. It is a really charming title, and if you are old enough to remember the original Super Mario Bros. you'll appreciate these awesome little nostalgic touches to the game.
Really, if you own a DS, there's not a reason in the world to not own this game. Highly enjoyable and recommended.