Decent concept, with a "is-she" "is-she-not" sort of plot device hanging over most episodes. But, even as a fan of classical music, I can't endorse this show more than, watch it if there's nothing else on.
The lead characters are, how do I say it,... far more "bland" than they need to be. I understand this is not an action movie. But you'd expect the characters to have some life in their eyes, or some emotion in their voices. Suzume sometimes does. But the other three never do. Unless it is for clearly comedic effect during a scene of such buffoonery that it feels totally out of place.
The story is slow. I don't mind slow. But compare this show to "Makanai: Cooking for a Maiki House" and you'll understand the difference. In that show, nothing much is happening, but the plot does move forward and the development of the characters is riveting. You want to listen to every word, and never take your eyes off the screen.
In Quartet, the characters never really say anything of importance, and repeat themselves so often, that you can safely fast-forward through whole sections of the show, simply because you know you won't miss any dialogue that matters in the least. Going into scenes, it is clearly obvious what will happen, and what will be said. There's no joy in that.
Bland, boring, and predictable. Could have been so much better. Casting and writing are the big culprits here.
FIVE stars for the music.