Some of the book's themes will be familiar to DWJ fans of old: neglectful or dead parents, overbearing older siblings, a central child character who gets manipulated by adults intent on their own interests, and that same child character having unrealised magical abilities (although this latter is, I was pleased to see, much less central to the plot than it is in Charmed Life, The Nine Lives... or The Magicians of Caprona). But none of this is to say by any means that she is simply re-treading old ground: rather, she's bringing in enough that is familiar to make the book recognisably part of the Chrestomanci series, while also introducing enough that is new and playing around enough with her own formulae to make it feel fresh and exciting.
On a very basic level, this is the first Chrestomanci novel to be set entirely outside Series 12. This wouldn't necessarily count as a literary leap in its own right, but it seems to come hand in hand with some new explorations of the concept of the related worlds, which are very welcome, and build nicely on ideas developed in Witch Week2. Can't really say what those explorations entail here: you must learn the full meaning and implications of 'pulling the possibilities' along with Conrad.
Another change is the use of the first-person narrative voice. This I liked because it gave DWJ greater scope for exploring both the confusion and the hurt felt at various stages by the main character, and also because it helped to make the prolonged tension between Conrad's imperfect understanding of what was going on around him and the actual situation (itself another classic DWJ device) all the more convincing.
Other notable features included some interesting playing about with the theme of acting (both in the regular sense and more metaphorically), a cast of consistently complex and three-dimensional characters and, of course, Diana's apparently effortless, yet rich and melodic, prose style.
All in all, I'm definitely glad I bought it: not just because I was right to believe it was a pleasure which should be enjoyed as soon as humanly possible, but because I'm proud to own a proof copy of a book which I now know to be truly excellent. I'll almost certainly buy an official copy once it is published on March 7th, as I very much feel Diana deserves my contribution to her royalty cheques. But I may just wait to do so until I've moved to wherever I go next after Belfast, as it's just silly to buy a book I've already read now, when I could wait until I've moved again.
 The complete order for me is now: 1) Charmed Life, 2) The Nine Lives of Christopher Chant, 3) Conrad's Fate, 4) Witch Week, 5) The Magicians of Caprona (although even that is still very good). Mixed Magics is unclassified, because I like some of the stories in it better than others, with 'The Sage of Theare' probably being my favourite.
 Although the suggestion in the last chapter that someone who stays too long outside their native Series will 'fade' is a) a bit reminiscent of Philip Pullman and b) not logically consistent with the amount of time Millie seems to have lived in Series 12 by the time of The Nine Lives of Christopher Chant.