It's classic 2D Mario in 3D.
To start off, I want to say that if you're looking for another SMW or YI, this isn't it. Sure, there are plenty of secrets, but it's not the same. The controls, however, are reminiscent of the SNES games. Unlike in SM64DS, Mario actually does what you want him to.
As for graphics and sound, both are great. The blending of 2D backgrounds and 3D characters is flawless. Everything looks amazing. The music in the game is catchy, and it's fun to watch the enemies jump or spin when certain sounds play.
The level design is the game's forte. While some levels seem a bit short, overall they are long and challenging enough. One thing to note is that it is practically impossible to get a game over. There are so many opportunities to gain 1ups, it's ridiculous. This doesn't make the game easy persay, as once I got halfway through the game, I was definitely losing lots of lives. This is particularly true when you go back to complete the game 100%.
This game is great not only because it's new Mario platforming, but also because of the innovation and diversity that appears throughout. Things you never thought possible in a Mario game or things you'd never expect in a Mario game appear in this game. Personally, that made the game that much more entertaining because I never knew what I'd see or experience next.
In closing, I highly reccommend this game to any Mario fan or DS owner. You'd be missing out on some solid fun if you missed this one.
P.S. The new multiplayer minigames rock!