The $20 DLC is available on Nintendo eShop now!
New Super Luigi U, a DLC for New Super Mario Bros. U, allows players to play as Luigi to celebrate the year of Luigi. Also, Luigi jumps higher than Mario, and player can team-up with Nabbit in 82 new stages.
Also, the game will be available as a standalone game in retails starting August 25th for $30. If you can wait, of course.
p.s. Club Nintendo is offering a special deal for those who download New Super Luigi U before August 1st. You can also enter the chance to win a Luigi Pin.