Closer to 3.5 stars, really. 
 I read this largely because of my fondness for The Time Traveler's Wife. The good news is that the author is indeed a very good writer. She has wonderful character development, solid dialogue, and her descriptions provide just the right amount of detail. Yet if your story is lacking, it doesn't matter how good you are at telling it. 
 GOOD: 
 * The writing, as mentioned above. 
 BAD: 
 * The ending was unsatisfying. I wasn't sure if it was just thrown together, meant to introduce a sequel (or both), or intended to provoke deeper thought. 
 * The pacing at the start of the book. It crept along for too long (though perhaps this aided the character development), though when it finally picked up 3/4 of the way through the book, I was extremely eager to keep reading and find out what happened. 
 UGLY: 
 * I am a big fan of quirky stuff in books. I am fine with weird as well, though this story and plot line did not appeal to my sense of weird. I realized this when I was explaining the last part of the book to my 12-year old, and how bizarre it all sounded. NOTE: I highly enjoyed the OCD-ridden Martin, in all his quirkiness and weirdness. 
 But what's it about? There are identical twins, who have been estranged for most of their adult life. One of them dies and leaves her flat in London to the nieces she never really met, provided they abide by many conditions. These nieces are mirror twins, and have an unhealthy dependency on each other. A few other characters are around, most notably Robert, the lover of the dead twin, and Martin, who lives in the same complex but will not leave his flat due to his OCD. 
 As it turns out, the dead twin (Elspeth) has become a ghost, trapped in her old flat. A secret permeates most of the story, even though there are enough clues given early on (and in the middle, and later) for the reader to surmise what happened. By the time the 'secret' is finally revealed, it felt anti-climatic to me. Oh, yeah. THAT. 
 (view spoiler)[ Much of the book revolves around Elspeth learning how to interact with the world in her new condition, including eventually communicating with her 'nieces' and ripping the soul of their kitten from its body. Things get creepier when one of the young twins, Valentina, wants to liberate herself from Julia, the other twin, by having Elspeth rip her soul out and then later putting it back into her days-old body. This does not go exactly as planned, and another twisted twist occurs. (hide spoiler)]. 
 Still, in the end I did enjoy reading it, and that is attributed to Niffenegger's writing skills. If you haven't read The Time Traveler's Wife, read that one instead. Or maybe read this one FIRST, and then you'll enjoy TTTW even more. :)