Jam with the Band is 100% music game. It doesn't have the flash of games such as Dance Dance Revolution but makes up for it by giving each player: beginner or expert a challenge.
JwtB could be compared to Rock Band as in both games you choose which instrument you want to play before you start the song, each instrument has it's own patterns to follow. It's soundtrack consists of a lot of different genres and new songs can be downloaded through wifi.
JwtB's magic happens when you get a couple of friends over and each take up an instrument to accompany each other. Only one copy of the game is needed to play with a maximum of 8 players.
I have played the original Japanese release for a few years now and I can say that I haven't had as good a time playing any other DS game. Some players may find the lack of flashy graphics to be a bad point, for me it has shown that a music game with a good gameplay and a fantastic soundtrack is enough to keep you entertained for months.