Basically, it's Donkey Konga on the DS with a plot, one which ranges from wonderfully absurd to heartwarmingly touching.
It's fun, it's loud, it'll keep you tapping your toes all the time while you're playing. My only qualm with the game is that it absolutely demands to be played on a TV with an audience. Much like Wario Ware, this game just screeeeeeeams to be watched, which is why I'd sacrifice a goat to see a Rev version (and a US release would be awesome as well).
I will, no doubt, be encouraged to go through to ensure that I get an A on every song (or S, if the rating is possible).
To summarize the gameplay, numbered circles will appear on the touchscreen. Around each of these numbered circles is a thin ring which will gradually close around the circle. Your job is to tap the circle when the ring connects with it. The numbers indicate combo chains which must be done in order. In addition to taps, you have slides, which will require you to tap a point, then slide the stylus along the slide (which will appear on the screen) all the while keeping the stylus within the circle which is indicated as you slide along the touchscreen. Often, the circle will reach the end of the slide and bounce back, forcing you to move the stylus back down the slide repeatedly.
The taps and slides will appear on the touchscreen in specific patterns and combos, all in perfect harmony to the music and the Ouendan "cheer squad" which is doing what resembles a cross between a dance and a martial arts routine in the background.
It's all in Japanese, but it's quite easy to glean what all of the functions are, as well as what's happening in the stories going on on the top screen. Also, after beating a level, you can hit the option in the lower left corner to replay it and duplicate your score so you can watch the story on the top screen instead of playing.
Total of 15 songs with 4 difficulty modes each (easy, med, hard and insane), and I've yet to beat the final level in med, which features "Ready, Steady, Go", one of the many intro themes for Full Metal Alchemist. I recognize some of the Japanese songs from the J-Donkey Konga, but most of them are new.
I only have one cart, but it also offers co-op and vs. multiplayer (multicard, of course). Overall, excellent game and the only successful concept in a rhythm game I've ever seen on a handheld yet.