The movie is hard to rate. I for one can't tell if there is some hidden Korean cultural context that I missed, or if that it was faking it. I am leaning towards the latter.
The movie is about a "soul band" in the seventies called "The Devils" and their existence in post war South Korea. You can in fact apply the formula that most band movies have to this movie. Without spoiling too much, it does have the standard things come full circle ending.
Possibly the biggest problem with the movie was the music. Used are various 60s/70s/pre-disco era American music, which is fine considering context, but the selections aren't great when couple with the singers, and the songs contribute nothing to the plot. In fact the entire setting is unnecessary, and could have easily taken place in modern day. This makes any importance that music had at that time in South Korea seem trivial and detaches the viewer from the experience of the film.
Acting was fine, if a little bit forced. The script wasn't great, but production was strong and direction was solid. The movie as a whole just didn't seem to come together very well.