This is the best game I have played, but it has it's flaws.
A cursor rolls through the bars in the song and when it goes through a note, you press the corresponding button, if you are playing on medium or higher.
It can be a pain sometimes, when you are playing on master difficulty, but, don't worry, you will get better with practice.
I can play a 4 star drum track in beginner and medium.
About the songs available in the game, that's the point that grades the game down, like Slave 4 U by Britney Spears, I hate that chick. Puff Daddy's Every Breath You Take was a great addition, it's a cool song.
Smoke on the Water, damn, how many times I have played this song, but it doesn't get old, it is a great song by Deep Purple. I added some songs to the game, please look at my GameSpot blog to see the list and feel free to comment on it.
I should say that I first used DS Download Play to play it with my friend, I liked the game and bought the cart, registered it to Nintendo and now I can download songs for the game.