So what i am about to tell you is a spoiler so do not scroll down if you do not want to know what i am about to write. However, know that below is not really a spoiler, to me its more of an incentive to get the game and beat it to completion.
Here it is,
After you beat the game and get all 120 stars, go beat bowser again, once you do that the game will reload however, to your surprise you will have unlocked luigi and be able to play the whole game as him.
F-n Sick is it not.