Amazing cinematography, editing and acting from the two leads, but the direction the plot took at the end felt really wrongheaded, conflating drive with talent:
For most of the film, it runs with the premise that Fletcher's teaching is damaging and destructive, and Fletcher's final humiliation and destruction of Neyman by choosing a piece he doesn't know seems to confirm that he's not being cruel to get results: it's a petty and vindictive move which, as far as knows, can only damage Neyman's career. Then the film takes a complete left-turn and goes for a crowd-pleasing triumphal ending where Neyman gives an amazing performance that the film really pumps up with fast editing and loving cinematography.
As a side note: Would any of the spotters in the audience really want Neyman? From their point of view they've seen him completely mess up a song, abandon the performance in tears, then turn up again, interrupting the conducter and forcing the rest of the band to play the song he wants, including a showy three minute solo. Despite the technical level of the performance I think they might be left with the impression that he wasn't entirely stable or much of a team player.
The film has been compared to Full Metal Jacket with drumming, but that would only have been the case if it had ended with Private Pyle going to Vietnam and kicking ass with his amazing marksmanship.