I didn't expect to like the music in this movie. I only watched it because it was directed by Allison Anders. Her "Gas Food Lodging" and "Mi Vida Loca" are among my all-time favorites.
The title suggests a movie about Johnny Cash, whose music I never cared for. I've never had much interest in June Carter or Jewel either, but in this movie I absolutely loved Jewel singing as June Carter. And she was so beautiful! Watching her sing was as much a pleasure as hearing her. I don't know whether she resembled the real June Carter, but she looked like a superstar, which I guess June Carter was in her time. She was Elvis' girlfriend, and she married Cash when he was the biggest star in the music business.
Even better was the group of actors singing as the Carter Family. I like the old Carter Family recordings, but the actors in this movie sound like a much better group from the same era, especially Michelle Kabashinski singing as Sara Carter. I'd love to have a CD of her singing old time songs in that style.
I can only think of 3 or 4 movies whose music I enjoyed as much. The rest of the movie was sometimes enjoyable and sometimes tedious. I'm sure I'll watch the music scenes again many times on DVD, but probably skipping past all the rest.