Shriver's use of a parallel universe structure is masterful and makes the book quite interesting. In fact, if she had not written the story in such a manner, I don't think the plot would have been as worthwhile--it would have just been another tired "should I or shouldn't I?" morality play.
My favourite aspect of the parallel universe structure was watching how characters react differently to Irina or to other characters depending on what choices she makes. For instance, the meeting between Ramsey Acton and Irina's mother goes horribly in the first universe (even though he buys her a car), but in the second universe, Irina's mother is so intrigued by Acton that she flirts with him!
This is the first I've read of Shriver's work, but her style and voice really drew me into the book. She makes some very poignant observations about life and the struggle to maintain--or endure--long-term romantic relationships. I sympathized with the main character, up to a point. Although I get that the point of the parallel universe structure was to play out two parallel possible lives without judging one superior to the other, I admit I took a side, preferring the universe in which she does not cheat on her partner.
Fortunately, it didn't matter that I picked sides, because it turns out that pretty bad things happen to Irina in either universe. Shriver's theme seems to be that "no matter what choices you make, your life is still going to suck anyway, although you can cling to the illusion of happiness if that's what makes you feel better." As a result, I can't help but get this feeling that Shriver forgot the fact that if two universe originate from a single point, they should be divergent. I realize that by keeping them parallel she allows us to compare Irina's choices in remarkably similar circumstances (for example, in both universes, Irina writes and illustrates her own children's book, it gets short-listed for an award, and she goes to the awards dinner). However, by unifying the two universes at the end, Shriver creates an ambiguous, postmodern ending that I can't enjoy.
I was ambivalent about giving this book two or three stars. Although I didn't enjoy the ending, the majority of the book was interesting and rich. Ultimately, I'm giving it three stars because Shriver is a good writer, even if I didn't end up liking her conclusion to the story.