For some real discussion: Has anyone sat down and watched a group of two to four five-year old kids play/interact? What do they do? My niece when she was five couldn't play Wii Sports except boxing (which she demolished everyone at) and cannot play Mario Kart at any competitive level (but she still likes playing Mario Kart, how odd). I asked her why she like to play Mario Kart and she said "it's fun". Same thing with boxing, she likes it because it's fun.
Without trying out Wii Music I do personally really wonder what kind of experience I'm going to get from it. I'm also wondering what my now six-year old niece will get from it. If kids like it, the 5-9 year old group, this will be fantastic. It will make babysitting so much easier. I can already see them making their own game with their own rules within the toy (Wii Music). I think they may use the toy to play a game. What a novelty.