This was a tough sell any way you look at it. I mean retelling the last books story, and from the POV of a teenage girl? It's bound not to impress some, and disappoint others. The retelling of the Last Colony's story was what really got to me at the beginning. I mean going through the same motions, from a different perspective, seemed like a waste of time, and like my views on the Ghost Brigade, I wanted to get though this one quickly so I could continue the main plot in the series.
But somewhere along the way, I started to feel like the story complemented instead of retold. Thankfully the latter half of the book was different enough in it's stories that you forgot about the main plot, and focused on the here an now (within the book of course).
Yes, the main story was spoiled by the previous book. I internally debated whether it would have been better to write a single book with two perspectives instead of splitting it into two books. But that book would have fallen under the same paradigm, and I can see people commenting how much more interesting John's story was compared to Zoe's. I'm pretty sure, that at one point or another these two books were though of as one, and then Scalzi, decided to split them up. There are several plot holes in book 3 which get resolved in 4, that I can't see him leaving like that, unless he already knew book 4 was going to happen when he was writing book 3.
All in all, I'd say Zoe's Tale isn't the best book in the series, but a good read none-the-less.