I think it will sell well (most first party titles do!), but I won't be buying it. I felt embarassed watching them play it on stage at E3. I got the impression that you are simply pressing buttons or using motion control to trigger preset notes, so you can't play the wrong note and therefore there is no actual "game" element to it at all. It would be like if Guitar Hero only used one button that always played the correct note and all you had to do was press it at the right time - but you get no points or have to face any kind of challenge. I could be completely wrong, but from what I've seen and read about it, that appears to be the way it works.
I have no problem with them releasing the game if there's a target market for it, but don't forget about the long time Nintendo fans who don't want every game to be dumbed down for the casuals!
