Jonathan Holmes got a chance to demo Theatrhythm during PAX East, and was kind enough to film it for those of us who weren't there. If you enjoy rhythm games and love Final Fantasy, I'm not sure why you wouldn't buy it.
I don't own a 3DS. There have been a few games that have seriously made me consider buying one, and I think Theatrhythm might be the game that seals the deal. Or maybe it's Holmes' singing. I do love some handheld rhythm games though, like Elite Beat Agents or DJ Max. And, of course, Final Fantasy. By my own logic I have to own this game.