I finally caught up with this lovely musical after seeing it on various 2007 best movies lists. This movie works for all the reasons that made Sweeney Todd such a bore. Most importantly this movie works, because the musical numbers work. It's a simple story, with almost no specificity and flourishes, but the music supplies all the emotional nuances we need to understand how Guy feels about Girl (yes that's the actual character names in the credits).
All of the musical numbers are set up in a way that's antithetical to what we're used to in standard musicals. There's no breaking the third wall, and characters going into song & dance without explanation. The main characters are both musicians, and we're essentially sitting in on various practice or recording sessions. In fact the whole movie is consciously antithetical to the conventions of the musical genre. All of this works to the movie's advantage, but honestly I'm hesitant to praise it for its convention breaking ways. Obviously musicals have been in dire need of modernization for a long time. But I still feel that this genre can be reinvigorated without completely throwing out all the things we love about it. I'll keep waiting for the movie that both modernizes AND pays tribute to all the great classics, and allthough Once is not that movie, it's still a great one and a must see.