I got to play this a few days ago when Mayhem's order arrived from Lik-sang while I was out there visiting, and HOLY CRAP is it a blast!
Nice mix of music ranging from J-Pop to Mambo No. 5 to some Mario remixes...including the rap from the intro to Donkey Kong 64! We didn't have time to figure out what all the options and gameplay were about since everything was in Japanese, but from what we had messed around with it was still pretty damn fun to play.
It's pretty easy on the easy and medium levels, but once you hit the harder ones, good luck! Even with only four "things" to do (left bongo, right bongo, both bongos, and clap - yes, clap) it still gets insane after awhile.
Multiplayer is definitely the way to go though, with each player getting a separate "track" of the song to play along with to make things a little more interesting.
Anyway, if you can play Japanese Gamecube games, get this thing NOW.
Kristine