It's been awhile since I've read The Last Colony and, when I started reading this book, I thought "I've already read this, but since I don't remember everything, no damage in reading it again". And somethings were different, so I thought "Well, my memory is not the same anymore, it seems." And when I finally finished the book and read the author's commentary about it, I finally realized I didn't read "Zoe's Tale". At all.
Instead of going forward with the tale of Old Man's War main protagonists, Scalzi starts at the very start of "Last Colony" and tells the same story by the eyes of another character, this time (wanna guess?) Zoe. Kinda like [book:Shadow of the Giant|8647 (and its series), which follows Bean instead of Ender.
It's a fun book -- I still love the way Scalzi writes -- but if you try to read after reading Last Colony, you may think you're reading the same thing again.