Me, I don't get it at all. Why would I want to watch a movie about a celebrity, Johnny Cash or Truman Capote, when I can listen to one's music or read the other's books? Heck, one year not long ago you could read two fictional accounts of one Hollywood starlet's life written by tweo celebrity writers. (Know who?) And why would anyone want to do that? Celebrities (known for being known) writing about and acting as other celebrities and reducing their targets to their own celebrity status: 'known for being known'. 'Nuff already!
I just saw the movie, and it absolutely blew me away.
I love country music, and grew up with Johnny Cash playing in the background. My parents' generation thought he was all it -- men especially liked his music and his style. He was already an icon in the 70s, but many of us youngin's didn't really see the appeal -- why was he so big? As I got older, and started listening to guys like Cash, George Jones, etc... with adult ears, I got it.
Yet, even though I went into the movie thinking I "got it", I was totally unprepared for what this movie would do. Phoenix absolutely captured that quiet half-swagger and the way that Cash was (on-stage, if not always in real-life) a man's man -- gritty, intense, edgy. No soft crooning going on here.
I realized after watching the movie that I had listened to Cash on AM radio, watched him on TV's with a 3 inch speaker, and even the records of the day didn't begin to approach what it was like to hear live performances.
This movie captured what it must have been like to be there when Cash was in his prime, hitting the stage like something no-one had ever seen before.
As to the religious bit, that is nitpicking. This was a great movie.