When I went to see this film, I didn't thought I was going to see a masterpiece, I expected to see a documentary on JS. Bach. Well, I must say I got it.
I found the actors rather bad (except for a few exceptions), Jean Rochefort (whom I do like) is not as convincing as expected, the quality of the image is rather poor (well, do bear in mind it's a small French film), there's nothing special in the filming itself, and if you don't love music you'll get bored very quickly because of long "concert" scenes. Things have been done rather quickly and even though the costumes are good, all efforts to make the spectator move to the XVIIIth century have not been made, we can even see some cars in the background of a scene!
The worst point of this film is music/image synchronization. It's really obvious that actors are not playing the music (except in one or two scenes). Well, I'm a pianist myself so maybe as a non musician you won't notice anything. But when Bach conducts the orchestra and his beating with his arms is not synchronized with the music, I guess even non musicians could get annoyed.
You can't compare this film to others on the same theme (musicians' biographies) such as "Amadeus" or "Immortal Beloved" (that's on Beethoven), those are films while this one is a documentary. After all, if you're interested in Bach's life and too lazy to read a book about it, then you could go and see it.