I loved this, it combines a lot of compelling elements to create an immersive story:
- the little beach town setting that, if you throw in the right amount of kind of sad elements, makes it easy to envoke a sense of nostalgia for something you might not have even experienced yourself
- a beautiful writing style; it's not poetic and I can't really explain why I liked it so much. It just felt well said and matched the tone of the story itself well. 
- a soft magical element, in this case the invisibility, which was the perfect one for a story about the struggles teenagers and especially teenage girls. (To me the wish to not be percieved and to escape through being invisible felt very relatable at least.)
-and last, but most importantly: the relationships between these four girls; their friendships felt so real and wonderful, the bond felt authentic from the very first second (I remember how Callie tells the reader how she and Talia met Cleo and how the summers went from then on. A lot of chemistry between these girls was simply transported by choosing the best words to transport this feeling)

Throughout the story, you always have this little sense of unease, of thinking that something will hit you hard at some point. But this feeling just simmers in the background, because this cozy summer setting tries to make you feel at ease ... well, it did hit hard in the end. 
I see how you could read this book and not be that affected by it, maybe saying that it doesn't go deep enough or moves too slow in the beginning, but to me every single choice the author made was effective.