Very enjoyable for a teen book -- the 16-year-old protagonists make everything Incredibly! Dramatic!, but once the reader remembers what it's like to BE 16, that seems very believable and realistic for the characters at hand. The Second Summer of the Sisterhood is, of course, a sequel to The Sisterhood of the Traveling Pants; Brashares does a nice job of explaining the backstory without being so heavy-handed as to undermine *this* story.
Characters are well-developed; conflicts are resolved in ways that are true to life (though not always pleasing). Some serious themes are addressed -- identity and family, the nature of true friends, and true love lost, among others -- but the book is never heavy or dark. It's a great snapshot be being 16.