I loved this series! The pace was perfect, the tone appropriate, the characters absolutely believable and realistic (a trait which I feel is critical in enabling a younger reader to relate to the story), and the language was lively and age-appropriate. The interactions between the mother and the children were as realistic as if they were sitting in the room next to you. The story, while not as creative as Fablehaven, included well known mythical characters (goblins, fairies, unicorns, trolls, etc) and didn't become heavy with overcomplicated mythical/magical elements. While I enjoyed Fablehaven, my biggest beef with it were the characters that did not act real enough (the main female character, for instance, dies, and no one bothers to inform the parents). And the magic was so intertwined and complex it was difficult to keep them and their magical abilities straight.
Another thing about this series that must be mentioned. I love the size, texture and illustrations/artistic additions to the book! There were just enough illustrations to paint the right picture at the right time for effect. And the book size is 4x6 inches, just enough for an adult to cradle the book in their hands and a child to caress and hide in their favorite backpack. I love books like that. Simon & Schuster thought of everything. There is an eye-catching colored glossy picture at the beginning, a treasure-looking map of the story area, and roughly 3-4 half/whole page black/white illustrations per chapter.
The length of the book is also age-appropriate. As an adult reader, I read all five books in one setting. A child does not have to invest too much time and attention to get a satisfying reading reward. I'm not surprised at all that these books were made into a movie. I have not seen the movie yet.
I read this series just immediately after reading the Lemony Snicket series. I liked these books much better. At the very least I wasn't inclined to smack all the adults in The Spiderwick Chronicles. The Spiderwick adult/children relationship is far superior to Lemony Snickets as I felt the Snicket's adult reductions to stupid/disrespectful/mean caricatures is not a good message to be sending children.