As a movie, I thought it was great fun and blended the story telling with the Kpop theme well. Soundtrack is excellent, though I wish they would have gotten the EXO song played in the movie in the soundtrack too.
My one qualm is that, for the concept being "three amazing singers use their voices to hold back demons," it was "one singer uses her voice and the other two get one line each and stand next to her." Even the most "maybe your faves should get good" Kpop fan would call out that wild imbalance. So that bit didn't work for me on either a story level or as a Kpop group level.