This review will contain spoilers!
In many sentences:
I really enjoy reading Diana Wynne Jones. She has a whimsical and magical quality to her writing that I don't remember seeing anywhere else. Her characters are all very well realized and entertaining (even when they are frustrating). I've read the Chrestomanci series completely out of order, but that hasn't interfered with my enjoyment of the books.
In terms of characterization, I felt like I better understood each of the characters in this book within the first few pages than I did the characters of the last book I read (Giant Thief) in the entire work.
I also really enjoyed the way magic is more the backdrop of the series (as is the parallel worlds), rather than the focus. It isn't a book about magic, it's a book about great characters, who happen to exist in a world where magic is real (and taken for granted). As with the other books in the series, this book also has an excellent ending, and I'm always a sucker for a great ending.
After finishing this book, my sister-in-law read Charmed Life, the "first" Crestomanci book. She didn't enjoy it as much as I did, annoyed as she was by Gwendolyn. Personally I enjoyed the book, but I think in all ways Lives is better. The action and excitement happens earlier, and there isn't the annoying whiney character equivalent of Gwendolyn. Also, the parallel worlds play a larger role, and are so clever. I love the way Jones imagines and uses them within the book.
This is definitely a book for younger readers, both in terms of themes and content, but I think anyone who enjoys whimsical adventure stories full of warmth and humor will enjoy this (and most other books by Jones that I've read).
On a completely unrelated note, this book has made me realize that there aren't any good books (that I'm aware of) that are set in an American location where magic is simply accepted and taken for granted. I think we need more of that - something that analyzes and investigates America, its history and culture, centered around a world where magic is simply taken for granted.