Yes, the film is a bit over the top. Yes, it is corny and sentimental in several instances. And yes, it does contain several stereotypes and cartoonish portrayal of Germans. However, despite all of these failings, the film is very successful for one reason; authenticity. The film is authentic because it was made smack in the middle of the German Occupation of France. The emotions portrayed by the French in this film are as genuine as one can get from a film.
George Sanders plays a lower-case Schindler in the film, and does a very good job, despite having to play a good guy (he is so much more effective at playing cads, neer-do-wells, and unfeeling characters). Brenda Marshall does an outstanding job as the lead actress, and Philip Dorn is very effective in his role of a lifetime as a returned POW.
The film does skip over one or two important elements of Vichy France, however. It plays up the resistance very well, but it does not really show how many of the French (Vichy Government) collaborated with the Germans. The single exception is an Italian barber, but Luigi is obviously not French (it is a sly slap at the Italians for being allied with the Germans). Luigi, to be sure, is a lowlife, but there were several thousand French lowlifes as well that supported the Vichy government. There are several good dramatic moments in the film, and one instance of selecting the lessor or two evils over the impulse to let a Nazi officer die. Compared to the dozens of other "French Resistance" films made since then, this one is easily in the top ten.