I have very mixed feelings about this book. Diane Duane is of course a great author and everything she writes is well worth reading. That said, I really didn't find myself enthralled by this book as I have by others in the Young Wizards series. It was a good enough read and I enjoyed it. But that essential "magic" that sets it apart from the average fantasy book was somehow missing.
My personal hunch is that the series has just gone on too long. The first three books were amazing, mind-expanding, earth-shattering high fantasy at its best. Here we had a couple of teenagers pitted against the evil Lone Power, and the fate of the world, New York, or the entire universe at stake. These books were exciting, enthralling cliffhangers that you just couldn't put down. Even now, chills go up and down my spine when I read certain parts of Deep Wizardry (probably my favorite in the series.) The first three books culminated in the defeat and redemption of the Lone Power at the end of High Wizardry.
The mistake, I think, was in continuing the series. After High Wizardry, the "high fantasy" aspects of the books began to disappear, which was quite understandable. I mean, how much further could Duane possibly go in that direction after the epic battle between Dairine and the Lone Power at the end of High Wizardry? (A Wizard Abroad attempted to retain the "epic" character of the previous three, and did not succeed, IMHO.) Instead, Duane had her characters turn inward, dealing with themselves and the issues in their own lives instead of saving the universe. The fifth and sixth books are very introspective, and as character-development novels they are very interesting. However, as fantasy they are lacking that essential tension that characterized the first three books. In High Wizardry, the fate of the universe was at stake; in Wizard's Dilemma, it's the fate of Nita's mother. (Not to say that this isn't important in its own way -- it just doesn't create the same atmosphere of nervous excitement.) In the first book of the series, the characters explore an alternate universe; in the sixth book, they spend most of the time in a single person's mind. The focus of the series has shifted from the macroscopic, cosmic, and epic, to the microscopic, introspective, and personal. And while nothing is wrong with this in itself, it's not what I look for in a fantasy book.
Another problem is that not as many new ideas are being introduced. In the first three books, the characters were still discovering their own magic and all its aspects. There was an atmosphere of exciting discovery. In this last book, however, the characters have adopted a "been there, done that" attitude to magic. Magic has become routine and predictable; while in the first two books the characters were out discovering new universes, here Kit uses his skills to...fix his VCR by persuading it to work with his TV. (Yawn.) Magic somehow just isn't *magic* anymore...
In summary, though this book is a good piece of writing, it is somehow missing that essential spark that would give it life, like the "dragon's eye" mentioned in the Wizard's Dilemma. However, the book is well worth reading and was highly interesting in several ways (such as its portrayal of an autistic child) if not as exciting as I hoped.