For all of the people complaining about spending money on this game: YOU DONT HAVE TO PAY! This game is very playable without paying anything all you get from buying it is a few extra levels and some different guns.
This is a great idea for a game but unfortunately the execution isn't equally awesome. Most modern song are the same volume level if they are mastered "correctly" (They shouldn't be the same volume for sake of dynamics but they are because that's what sells). Also, the tempo tracking isn't very accurate.