I love Isabel Allende's writing: I read Daughter of Fortune and House of the Spirits, and found this one good but not quite up to the quality of the other two; lacking a distinct plot of its own, it mainly serves to tie the other two (standalone) books together. Still, it was an enjoyable book: Allende's character development and writing style are excellent as always, and I'm in the camp that loves to use fiction as a way to learn about the history of other countries, which this one certainly provides.
Still, the structure was somewhat odd. I would describe it as three novellas tied together to make a novel:
1. The continuation of the story of Eliza and Tao Chi'en (and company) from Daughter of Fortune. On the positive side, we finally get that love scene that went missing from the previous book; on the other hand, so many beloved characters come to tragic ends that I'm not sure bringing them back was a good thing.
2. The backstory on Severo and Nivea (and company) from House of the Spirits. This was my favorite part of the book: I liked them in House of the Spirits and like them even more now, and as it turns out, there was a lot we didn't know! This is also the plotline where we get to read about 19th century Chilean wars, politics and such.
3. The story of Aurora, the narrator, who does not appear in either of the other books. While not as well-developed as many of the other characters, she's the glue that holds the rest of it together. Her life is interesting enough, but the truth of the Big Terrible Secret from her childhood is obvious from quite early on (perhaps intentionally so), and she lacks the strength and character of the heroines in Daughter of Fortune and House of the Spirits. I didn't think that setting her up as the narrator worked very well; she winds up describing in detail conversations that took place before she was born, other people's sex lives, etc., and generally stretching credibility.
Others have mentioned Paulina del Valle as the main character, and I think there's some truth to that (although I see her in more of a supporting role): she raises Aurora and is around for most of the book, and certainly has the stronger personality of the two. She's one of those trademark Allende characters who make fascinating character studies, but are hard to like.
I would recommend this book, but primarily to those who have read some of Allende's other work and liked it; if not, I'd recommend starting with House of the Spirits, which is her masterpiece. Come back to this one once you've read that and Daughter of Fortune and are looking for the rest of the story.