As the director says on the special features of the DVD,

The film is not about the script as much as it is about the music and the way the music affects the viewer. The director (John Carney) made the choice to use musicians as actors because for him it was more important that the songs were well performed. He even used tricks to make the performances better, such as filming from far away so that the two main actors weren’t aware of the camera. It works well though. It was also made in a documentary style on a very low budget. The style somehow manages to make these two people become real to the audience. Like it is a documentary about their friendship rather then a fiction film.

I personally really like the songs. They were mostly written by Glen Hansard which really helps his performance. He is very much in the moment when he sings and I always like to watch people perform their own songs. The film is very unusual and extremely subtly and slowly told. I thought Once was a very beautiful film. It felt a bit like watching a very long music video with a bit of a story in between the songs. And it was very refreshing to see real looking people on screen and not polished Hollywood stars.