Great book for anyone with a dissociative disorder, or anyone living with someone with a dissociative disorder. Since it is written by two psychologists, one of whom actually has DID, it makes you feel a little less crazy and a little less alone.
I think the biggest drawback is that it focuses entirely on the most extreme form of DID, whereas dissociative disorders are far often more subtly presented than that. For example, not everyone with a dissociative disorder has such distinct personalities that switch back and forth. Often the lines are less clearly drawn.
Also, I feel that it is detrimental, at least for me, to give alters different names and clearly drawn identities, an exercise suggested by the authors. To me, doing so would make it even more difficult to integrate those aspects into one being, because you now see them as their own, individual persons. But they're not really, at least not to many people with DID. They're simply aspects of yourself that were created by your own mind to handle trauma. I feel that giving them individual names and identities will make it even more difficult to accept them as all part of ones self. So I call mine "angry me" or "child me". Because they're all still me, in the end.
I've started reading Stranger in the Mirror too, by M. Steinberg, which I think is a great compliment to this book as well.