I'm a bit of a sucker for this style of book, one which explores the what-ifs of life. Maybe we've all wondered what would have happened if we'd done things differently? Take one decision and life unfolds in a certain way. Take another and things are completely changed. But, to be honest, I didn't find the alternate 'lives' presented here to be different enough to be really compelling, and I didn't like the 'happy families' ending.
The starting premise is that one day Rose and Luke argue over whether or not they'll have a child. And in different time-lines, the fall out from the argument is different - sometimes Luke leaves, sometimes Rose does, sometimes their relationship carries on - but the slightly disappointing aspect was that whatever happens somehow having a child to love and care for (even if not her own) is seen as necessary for a happy-ever-after scenario for Rose.
Generally I didn't find Rose to be a character I could sympathise with. She made sure from the outset that Luke was aware of her decision to never have children, yet it hardly takes any persuading from him for her to consider changing her mind. Also, when there's surely no need to choose between career and having children these days, Rose's main objections didn't make sense - it was as if the author didn't want to follow the thought path that not everyone likes children or wants them. I'd have enjoyed it more if in at least one version Rose had thrown Luke out, and led an exciting, fulfilling, childless life.