For an American viewer, at least, it's a triumph of ambience: you get the sense that you're tucked into a fully slice of Parisian life, so vivid that you make it part of your (necessarily stereotypical?) picture of what France must have been like, at least in its time. On a quick look, you are tempted to bracket it with the Maigret mysteries of George Simenon. There are some huge gaps in the comparison but it is not entirely wrong. They both give you a world that is sordid and mean at first look, yet peopled with an array of memorable characters who seem driven by an unspoken compact to maintain a certain kind of an order, and mostly endowed with an impressive knack for muddling through. These people have been here, as Proust might say, since Geneviève de Brabant; minor nuisances in the way of murder and betrayal are not going to derail them now.
So for an American viewer, a consoling confirmation of all preconceptions. Yet here is the odd part: forget about its international cachet, evidently the film is a favorite in France, as well. This strikes me as odd: whatever the French view of themselves, you'd hardly expect it to comport with the view of the untutored foreigners: especially those who are so misguided as to suppose that they actually know something about France. Or, I guess, the possibility is that the French have foisted it all on a credulous international multitude while they stay home giggling into their fromage?