I'm really ambivalent about this book. To say it's being distributed practically for free, I feel a bit guilty saying anything too harsh.
Basically, as another reviewer stated, you DO care for the characters and you DO want to know what happens to them. Even though they are at times inexplicably heroic, then inexplicably stupid, then have inexplicably Mills and Boon sex at the least appropriate time possible.
The idea behind the threat was interesting and it would have been nice to get more of a view of the history between Humanity and Sartan, rather than the hasty exposition that was crudely tacked on right at the end of the book. Having said that, the plot did require a certain level of mystery and the author carried it off well.
It's not terribly well-written to be honest, but then nobody is expecting Nebula Prize winning stuff here. It was a likeable story that had both ambition and imagination, so it gets four stars from me.