This movie is either too short or too long.
If it tries to follow a book and to show the whole life of several dozen people, it should have been made as a mini (not too short) serial. I still remember how much I've enjoyed first TV version of The Forsyte Saga, made in 26 episodes.
There are also far too many characters in the movie. (I know France is a big nation, but they didn't have to show all of them in one movie.) For the first hour you even don't know who the main characters are (unless you've read carefully opening credits). Later during the movie some of them never appear again, some appear when you've already forgotten who they were and you don't care for them any more (as well as main characters and probably the director himself). Some get a significant footage in certain part of the movie and then never show again, being completely irrelevant to the plot (or having a subplot of their own that never develops). Yes, life looks that way, you can suddenly meet a person you haven't met for ages, but life lasts decades and you can't compress it into 180 minutes.
The movie promises very much in first hour (though this extreme number of characters obstructs your attention and complicates following the plot - and sometimes you wonder if there is any). Ball scene (often mentioned in other comments) and some casual talking scenes are marvelous in best French tradition.
But suddenly, as if the director discovered that his movie should last more than twelve hours if he kept the same rhythm, we jump along the years and we have some important things just mentioned as if someone waking from coma now and then and getting a few basic informations before losing conscience again.
The final hour is the best, but I'm afraid many people haven't seen it, either because of giving up, or simply falling asleep while trying to find who is who and what is he doing. Even those with best attention, who could solve this two questions, had no chance to answer the third one - why. Maybe we, who stayed awake till the end, managed to understand the main characters, but it is not a compliment for a 180 hours long work.
Some people compared this movie to Visconti's works. I'd agree, as I find Visconti the most boring of all overrated directors (and, just to mention, I respect Tarkovsky, like Tornatore and adore Bergmann - and ignore action movies).
Except making a serial, this movie could have been made watchable in two other ways. First, it could be made without middle part - after 1900 events we could have skipped into WWII without losing anything. Second, Assayan could have made what Kazan did with Steinbeck's East of Eden - chose one part of the novel, one plot and cut away the rest. We could have lost characters like Louise, Aline and her friend (?), Fayet etc, but I couldn't care less for them anyway. Maybe someone would find it a blasphemy for the literature, but making people yawn and bore isn't a favor to it either.
