My intention is not to write a full review, but to propose a perspective on this film that seems overlooked. On one hand is the (not so) sophisticated art-house film lover who thinks this movie is a challenging French "coming of age" film. They view it as a comedy with likable, mischievous adolescents groping towards enlightened adulthood, and a warm, good-natured mother, with sparks of anarchic, libertine fun to break up the steady stream of philosophical musings of a prodigious (and therefore charismatic, obviously) adolescent protagonist, destined to be the next Sartre or Camus. On the other hand are the people who see the same thing, but are shocked and confused by things that strike them as, in fact, depraved and ugly. These latter people don't understand how, for example, a comedy could feature incest, and they wonder why they find it hard to like the characters. Both sides -- to me -- may have missed the point.
To me, Malle's film is not at all a "comedy", but rather a psychological horror film. It's a study of "creepiness", and clearly a very unflattering (even hateful) portrait of provincial bourgeois French society. It's a film about creepy, damaged (bourgeois) people doing creepy, damaging things to themselves, to their domestic help (who are tellingly portrayed in a very positive light), and to each other.
What I see is Malle's very obvious dislike for the protagonists in this movie. It's a film about awkward, unlikable, brutish, cruel, unfunny people, in all their glory. The kind of people that make you want to take a shower after spending just a few minutes with them. The protagonist son is awkward and ugly (in both style and physical appearance), brutish, and violent. Make no mistake: if you actually thought Malle was suggesting that mother-son incest could possibly be a sweet, tender moment between two healthy, happy people, or if you laughed when one of the arrogant, obnoxious children wantonly destroyed a Corot hanging on the wall, and if you perceived all of the violent, awkward treatment of the domestic help and women throughout the film as tender or comedic coming of age moments, then you really need to re-evaluate things. There's no way whatsoever that Malle intended any of the characters in this family to be anything other than detestable -- and it starts with the detestable mother, who damages everyone and everything she touches. So, for all the people who thought they were supposed to laugh throughout this film (but who wouldn't want to admit that it actually didn't feel genuine for fear of seeming less intellectual (how ironic!), and for all the people who were incensed at Malle for suggesting incest could be seen as anything other than physical and emotional violence, I think you may want to rethink what's at the heart of this film.