The self-explanatory title is one of the many indulgences of this film. There are few surprises here. Jesse James is alternately a bad man, good father, and psycho. Robert Ford is alternately a weasely hanger on and a calculating, cold-blooded killer. They spend the entire film in a sort of macabre dance, like a moth and a flame, ultimately destroying each other.
What worked: The characters are rich and deep and well acted. This history is palpable. The story is as sparse as the landscape and feels like that's as it should be. The color palate and narration give the work a feel similar to Ken Burns' epic "The Civil War". There is tension in every scene, which is the only thing that allows the deliberate pacing to work.
What didn't work: As I wrote above, this is a self-indulgent film. It often languorously wallows in its own sense of self-importance, perhaps a bit like Bob Ford. And while it inhabits the same vast landscapes as other Westerns, it seems to give that up for a claustrophobic fixation on the two antagonists.
Ultimately this is a good, but flawed, film. One must be in the right mood to tolerate its moodiness.