Hello there my dear friend.
Daigasso is my favorite DS game. It's got a lot of neat songs on it; video game music, japanese songs, and lot of songs we know in America and Canada, but I haven't played it in SOOO long that I kinda forgot most of it, or I dont remember their names. 
It's hard to explain how it works, but basically the screen scrolls slowly vertically, and there's like, 4 rows on the screen at a time, which are constantly scrolling vertically, and the rows consist of boxes (each box is like, the time segment for the music). The music starts when the row with the notes in it comes to the top, and there's a highlighter that highlights the box in which you are supposed to play. The notes consist of arrows, buttons, top keys, and sometimes a combo of top key+button, and you hold that note for however long the line is (ex, Y======== is a Y note that you hold for pretty long, and if it's just B without a line, you just tap the B note). Explanations do this great game NO justice, you should find a video.
There a a few different modes, but I really only did the main one. I think after you beat like, 3 songs in a row or something, you go to the next level, which is harder. The difficulty is pretty good; starts you off easy, and just gets harder and harder each time.
I have no knowledge of the expansion pack whatsoever. I tried researching it one day, but didn't find much info. One of my main gripes about the game was the lack of more songs, which I suppose the expansion pack would fix.
The game is definitely worth importing, especially seeing as it doesn't seem it's coming to the US anytime soon. You don't really need to know Japanese either, and there are guides anyways.