What I liked was the epic scale (the story covers 50 years, from the end of World War II to the mid-1990s), the historical and geographical details, and the byzantine workings of the espionage game. That's really where Littell excels — you feel as if you're reading fact not fiction. He clearly has indepth knowledge of what he's writing about.
Where Littell stumbles is with repetitive, cliche-ridden dialogue (eg: "This is turning into a fucking can of worms," The Sorcerer muttered. "I think we're barking up the wrong tree — we maybe ought to give some thought to taking our business elsewhere" or "There was someone once, but too much water has passed under the bridge.") and heavy-handed attempts at romance. It's immediately clear which women the spies will fall in love with…and repetitively done (every time a spy gets involved with a woman behind enemy lines, he falls in love with her and, if she survives, he marries her).
Littell also has a troubling fixation with female nipples, describing them often and usually in the same way. He has one woman's nipple torn off during a torture scene and, more than once, describes the nipples of the young girls who are being preyed upon by a child molesting spy chief.
But cliches and nipple fixations aside, this was one of the best spy novels I've read in years. Now I'm going to watch the TV mini-series to see how the adaptation was done. I am curious about the creative choices the screenwriters made to streamline the plot and pare the novel down.