Concept: Take Mario 64, put it on a portable console, add three more playable characters and some extras, deep fry, and tada. You basically unlock new characters while searching for each of the 100+ stars in the game, hidden in the paintings and around the castle, unlocked by completing quests and tasks.
Sound: The sound is what you would expect from a Mario game. Also, if you've ever played Mario 64 for the N64, you know exactly what to expect, with the addition of Luigi, Wario, and yoshi's sounds/voices.
Gameplay: Just as good as I remember. I prefer the standard controls, but you can also use the stylus if preferred. You can switch between each of the 4 characters, each of which have their own special abilities. I'd recommend looking at a guide so that you can unlock the other characters early on, thus enjoying them earlier.
Story: Princess Peach invites you to the castle, Bowser ambushes you and kidnaps the princess (who would have guessed?), and Yoshi comes to save the day and free the others to help him. Not a very deep story, but enough to entertain.
Replay Value: I'd recommend this as a purchase rather than simply a rental. This is one of the few adventure games that you want to play again after completion. Not to mention that a multiplayer mode has been added.
Overall: Just as good, and better, than Mario 64 originally was. This is "the" 3D Mario adventure game. You will enjoy this if you enjoy Mario at all to begin with. I highly recommend this as a must buy for the DS if you play the DS often.