I didn't hate this book, and I actually read it pretty quickly (although it is very short, so that's not surprising), but I just thought it was unnecessary. The only thing it did was make Sutton more likeable/human, but I don't understand why that couldn't have been explored in the main series. I definitely didn't need 80 pages of a separate novella devoted to that endeavour.
Although, let's be honest, I will read anything Sara Shepard puts out because these books are my ultimate guilty pleasure.