I also have to strongly recommend this game, especially for those who played Elite Beat Agents. The game is easy to play but insanely hard to master; I've worn out the touchscreen so much on my DS that it seems to have permanently shifted 2 centimeters to the right through frenzied stabbing of it with the stylus. So yeah, it s quite addictive...
Personally I prefer the second game as I adore the rival squad, but the first does have better songs, if you're into Japanese music. The characters themselves are probably the coolest I've seen in a video game, as they basically turn cheerleading into a manly, hot-blooded dance off. Yeah, and hard mode WILL destroy your sanity....
On a side note, to those who played the game: remember the third hidden song in the second game, the one with the samurai? I unlocked that at about 11pm, Christmas Eve, which was very strange...