If you have the choice between the series and the movie, then watch the movie. It has several scenes that are missing from the series and round out the story.

What I liked:
* I loved the two female characters! They did not have much screen time, but showed some interesting character traits nevertheless. The team leader is obviously under a lot of pressure to produce music that sells well, is a real career woman who can be ruthless at times, and at the same time she seems to want to produce good music as well. The deputy is similar, and she does care about the people who work under her, at least Sang Yi. It is a delight to see women who are people with their own ambitions, even if they are supporting characters.
* I liked Lee Sang's acting. He did a good portrayal of his crush in the beginning and his shyness to speak to In Soo.

What I did not like:
* The story was thin, very thin. The premise has great potential for more, and it's saddening to see that it wasn't developed more. On the other hand, it was a good decision to not put too much into the short time frame.
* The lighting was at times a bit too much on the mood lighting side.
* I wish the scenes the singers would actually sing what we hear, not just be shown as a bautiful backdrop to the music.

I am torn about whether there was too much music with pining stares and montages or not. On the one hand, it is nice that the story took its time to unfold -- and it is about two people who fall in love with each other's music first, and then with the person. If there was less singing, I would maybe have complained about that.
On the other hand, those scenes took away time that could have been used for more plot or character development. Sometimes I fellt like I was watching a music video with interspersed dialogue.

You can watch it if you have some time on your hands and can use a pick-me-up, but you won't miss much if you don't.