This is one of only a tiny handful of books which I have started and never finished.
For me, the main problem was that the blurb implied it's an espionage story. In fact, it's a literary work about French political thought and communism from WWI to the present day. If you're interested in that, or you're a fan of Hedi Kaddour, you might possibly enjoy it. If you're looking for an Alan Furst style read (as I was), you should probably steer well clear.
Out of 660 pages I read the first 400 and the last 50 (hoping that something interesting might happen), so I think I gave the book a fair trial. As another reviewer has pointed out (and wittily lampooned), it contains some monumentally long sentences. This stylistic approach is an attempt to present each character's take on the world as a stream of consciousness. We visit the characters at several different times between WWI and 1991, hearing their inner voices and chunks of dialogue (actually monologues with responses only being hinted at). It all makes for a heavy read and I honestly don't think I missed much by skipping those 100 odd pages!
Unlike most espionage thrillers, very little actually happens. To give you just one example, two chapters (roughly 90 pages) are taken up describing a dinner party in 1965 in which the main events are: a) one of the main characters flirts with a guest b) there's an argument between another characters and André Malraux, the (real) French writer, which I couldn't understand (I think if I'd known something about the history of French communism it would have helped) and c) a discussion about a kangaroo that may, or may not, have appeared (I honestly can't work out which!) in one of Malraux's books.
Fortunately I borrowed this book from our local library. Before you shell out and buy it, I'd make sure it's really the kind of thing you want to read.