The book is an enjoyable, light read, but not one of my favorites.
Partly, this is a matter of thwarted expectation: The first time I read it, I was disappointed to find that it was not as funny as Dark Lord of Derkholm--it really can't be, since that story has already been told, and the parody is over. Also, the book is more standard-issue than DWJ usually writes--leaving aside the magic, it's a fairly typical going-away-to college and beginning to grow up story--new friends, coffee, crushes on and quarrels with teachers, new love, pretty much what one expects from the genre (type? subset? What does one call a book-type that crosses the recognized genres?).
Of course, "leaving aside the magic" is a big leaving aside: The magic provides much of the book's fun as the students try new spells and casually vary the instructions to suit their equipment, using orange peels rather than chalk to form diagrams, for example, or building a pentagram of books instead of drawing it out--both of which have interesting results once the spells activate. And it is a delight to see Blade and Kit acting together as a trained team and to take a quick look to find out how Calette and Don are doing.
Do I recommend it? Of course. It's by Diana Wynne Jones, isn't it? Do I think it's her best? Nope.
For other views, see this review at Jenny's Books.