Alien invasion. But in this particular event, we, the humans are the aliens who have invaded the planet of the 'fuzzies' (though unaware of their existence initially). I have not read the original 'Little Fuzzy' by H. Beam Piper so I cannot compare this novel with it but I think that it was good to read this book prior to any reservations I might have had against it if I have read 'Little Fuzzy'.
One thing I realized after reading this book is John Scalzi is immensely funny, witty and daring. Hollywood has pretty much revolted the audience by hiding behind the CGI effects and thus corrupting the charisma of the original story but I am more than sure that John Scalzi didn't have the luxury of CGI to write this one.
All in all, a very funny but thought provoking book. Jack Holloway, the protagonist, is one of the funniest and smartest characters ever written for science fiction books.