Without giving spoilers, I will say that if you're expecting/hoping for a cheerful high school reunion--as I was--then leave this book alone. If you're willing to endure an emotional roller coaster that will leave you angry and depressed before you find relief, then go ahead and read it.
Since finishing the book, I've forgiven Ann Brashares for utterly breaking my heart. At page 75 I wanted to quit, by page 315 I understood, and after page 349 (the last one), I was glad that I persevered.
The story was not at all what I was expecting. I kept asking myself why Brashares bothered to write the book. Her characters are miserable and restless, their friendship is a mess and completely falls apart after a tragedy (I.e. they don't speak to each other for about six months and one of them even doubts whether they were ever really friends), and they are the worst versions of themselves as they deal with their grief in isolation. I got quite agitated that more than one of them fell into old bad habits that had crippled them as teenagers, despite it being ten years later. Fortunately, due in large part to Tibby's interference through a series of mysterious letters, everything works out quite well for them. There is a loose end in Carmen's storyline that I feel is easily filled in by the reader's imagination. In mine, she simply places a phone call to reconnect with someone that left a big impression on her near the end of the book.
I'm glad to have seen Brashares' vision of what happens to the sisterhood after the pants got lost in book four...and it was a cathartic read...but I'd have been happy holding on to the nostalgia of those books from years ago with my imagination free to do what it wanted with the characters, and somewhat wish I'd remained ignorant about the details of this story. It was an impulse pick from the library; I'm glad that I checked it out rather than purchasing it, and I wish it had come with a warning sticker along the lines of: "may induce tears," or, "may cause the desire to throw the book against a wall.". I was too angry most of the way through to cry; that's the only reason that I didn't. Once I understood what had happened, I calmed down and made my peace with the book.
Reader proceed at your own risk. Also, I didn't remember this much language in the earlier books, so between that and the sexual references (there aren't any inappropriate scenes, though), I would not recommend this for younger teens--the age that the first book was initially targeting.