As their generic names suggest, Guy and Girl aren't exactly complex roles, which is just as well seeing as how Hansard and Irglová have only one acting credit between them. Even though their limitations shine through in some of the movie's more emotional scenes, the two share an easy chemistry that sells the characters' unlikely friendship. It helps that writer/director Carney has conceived the story so that his stars can be musicians first and actors second. Some 13 songs are crammed into the movie's slender 85-minute running time in various ways, from actual performances to that old reliable standby, the musical montage. Perhaps the film's most inventive sequence finds Girl walking home from a trip to the corner store while listening to one of Guy's songs on a CD-walkman. Filmed as two lengthy tracking shots, this scene will resonate with anyone who has ever used their own personal music device to score a simple stroll down the street.

Because the story is so slight, Carney occasionally finds himself stretching to pad Once out to feature-length. The film's middle section is its weakest as Guy and Girl struggle to define the exact nature of their relationship. (This is the kind of material that might have benefited from more experienced actors.) Once they enter the recording studio though, both the performers and the director seem to be more at ease. For 48 hours straight, they do nothing but think about, discuss and play music, running almost entirely on the energy generated by the creative process. Guy emerges from that marathon session with a disc full of potential hits and Girl later gets her own souvenir as well in a final scene that could be dismissed as completely corny if it weren't so charmingly sincere.

Once may not boast stellar production values or elaborate dance numbers, but in its own scruffy way it captures the spirit of the genre better than any recent Hollywood musical.
— Ethan Alter