I picked this up on a whim, and read it cover-to-cover in a single day, which is unusual for me these days.
The style takes a little getting used to. The chapters are all very short-- 4-6 pages usually. The focus jumps from person to person with each chapter, for 7 characters. At first this seemed choppy to me, but by the time I was 100 pages in it seemed fine.
One of the quotes on the cover mentions "Twilight", which is misleading in a couple of ways. First, this book is not a romance in any sense; and second, this book is well-written as a piece of literature. (I say that having enjoyed "Twilight"-- but Meyer has severe limitations as an author.) "Buffy the Vampire Slayer" is also mentioned, and is also misleading; the tone of "The Circle", the themes that it explores, and the stakes that are played for, are all more serious and dark than Buffy.
The authors do a fantastic job of getting the points of view of the different characters across without being heavy handed. For example, a character will describe what she sees in the mirror, which you (probably) just accept as being accurate; it's not until chapters later when the same character is described by someone else that you realize there's some self-esteem issues for the first character.
This book succeeded in shaking me a bit-- there was a point where I got very uncomfortable, not due to any gruesomeness, but simply because the authors had earned my trust and then used it to punch me in the stomach.
"The Circle" has some of the most nuanced and realistic depictions of what it's like to be a teenager that I've ever read, and certainly the best in any Fantasy/Supernatural genre fiction. I think they could have kept my attention even if the setting didn't contain anything out of the ordinary.
This is a good page turner. It's probably not one I'll ever re-read, but on the other hand, I will pre-order the sequel, which I guess is a pretty decent definition of "4 stars".