So much fun, my head now hurts.
Sometimes the things you must tap seem to be in time with the lyrics, other times it's in time with an instrument or the baseline. Often, the physical arrangement and placement will help visually cue you about timing differences. Things that have require more rapid taps will tend to be placed more closely together.
While this is all happening, you have the EBA's themselves dancing onscreen, presumably using their elite dancing skills to aid everyday citizens around the part of the globe that seems to be the EBA's jurisdiction (looks like we only get one hemisphere).. The stories play out on the top screen, while the action takes place below. You will almost never have an opportunity to observe any of this, except during the interludes between musical sections, or by watching a replay (and you can save the replays for viewing later, or competing against in a "ghost" mode).
The song selection includes hits from the 70's through the current decade, including Village People, David Bowie, Madonna, Brian Setzer Orchestra, and Avril Lavigne, and several others. All of the picks are bonafide hits, so while not every track may appeal to everyone, there's a very good chance that anyone will be able to find multiple tracks they like.
There are four difficulty levels, Breezin, Cruisin, Sweatin, and Hard Rockin. Each difficulty level uses a different head agent, except for Hard Rockin, which uses an altogether different group of female agents, called the Elite Beat Divas.
The difficulty of this game is substantial. Many songs require a combination of memorization and a sense of timing, as well as a good deal of manual dexterity with the stylus. In general, the songs do scale in difficulty, saving the hardest for last. There will inevitably be some songs a person is better at, due to familiarity with the song, however the last song, Jumping Jack Flash, will be difficult even if you're Mick Jagger.
While the gameplay above may not sound like much, when you combine it with its difficulty and unforgiving sense of time, it's really challenging. The sound effects for success are very satisfying, and complement the song as a form of percussion. Each successful tap is a very effective mental reward that is likely to addict you to the whole process.
The audio is pretty typical for a DS game. It's not as crackle-free as midi tracks (like the music in Animal Crossing), but it's clean enough. It sounds similar to a low bitrate mp3. You probably won't be bringing your DS along in lieu of an iPod, but even at a reduced audio quality, playing along with legit pop hits on the DS is a treat.
Graphically, the EBA's are rendered in polygons on the lower screen, and are nothing to write home about. However, the stories and illustrations are very colorful and clean. Even though they aren't really a direct part of the gameplay, they increase the slick-value of the package.
The game's quirkiness and/or Japanese-ness may be offputting to some, but in essence, this is an addictive pure-gameplay experience to be had, with some comical and well-scored window dressing.