I tend to be skeptical of screenwriting books that are too prescriptive, because while I believe there are universal dramatic principles, I think there's quite a bit of latitude to how they can effectively be applied. For instance, does the "point of no return" have to happen by page 25? Really?
I like that she does admit some great films don't fall into her "Nutshell" structure, particularly the Coen Brothers' later works (including Fargo, which she admits to loving, but not understanding —at least structurally).
I also like how Chamberlain bases everything on Aristotelian Tragedy and Comedy, which does help make sense of her technique, so that it's not all entirely new terminology which has to be translated in my mind to fit into the jargon of other screenwriting gurus.
Her emphasis on the character's flaw as the engine of story is interesting, and I think she's onto something when she distinguishes a situation from a story by the ways in which a story tests a specific character's flaw whereas a situation is not a problem unique to that character. In other words, if you can swap your protagonist with another one and your script works just as well, it's not a story. I think that makes a lot of sense.
I like the plethora of examples in the book, a few of which are undeniable masterpieces that help prove her points. I do find it funny that in the case of Godfather, which I think is the greatest story ever put to film, she admits that it doesn't fit perfectly into her Nutshell stages. There's also something interesting about classifying Godfather as an Aristotelian Comedy, because in the world of the film's gangsters, Michael overcomes his fatal flaw of naïveté, and moves towards his "strength" of realism. I don't know if I agree that Godfather isn't a tragedy, but maybe there's something to the fact that it can be seen both ways that makes Godfather feel transcendent, whereas some films that fit neatly inside the typical formula can feel a bit morally simplistic by comparison.
Something to think about.