It's not too hard to dismiss this book out of hand as being too arbitrary or deal-a-mealy. I was tempted to myself, but upon reflection I saw that while it's not perfect, it presents several advantages to going it alone.
While it's certainly possible to produce a formulaic, cookie-cutter book by following Marshall's guidelines, it's not inevitable. On the other hand, if you're too anal in your efforts of following "the plan," odds are you'd have been too anal to produce a saleable manuscript without it, as well. While it may not result in providing you with a break-out novel, the plan isn't intended to provide that result. Better to break in first, then you'll have somewhere to break out of.
As with most things, the principles behind the plan are its soundest points. I think there's room for some leeway in the execution of Marshall's points. Your decision may be different from mine, but I'm inclined to give him the benefit of the doubt before I begin flailing around in the dark without a guide.
At first glance, though, parts of the plan seem to make no sense. For instance, in sections told from the (for lack of a more nuanced word) villain's p.o.v., Marshall says the villain never experiences failure. This doesn't mean, however, that the villain can never face a setback, but whatever setback he or she faces is resolved and turned back against the hero before the section is concluded. Likewise, I think it's fair that the hero can stymie the villain in a hero section (or the confidant/love interest in their sections), but he/she/they/it needs to recognize the inevitable repercussions for what has been done (publicly insulting Snidely Whiplash may make your hero(ine) feel good at the moment, but upon further reflection (s)he ought to have doubts about the wisdom of those words/actions).
The only thing that has left me scratching my head is that Marshall has would-be writers determining the word count of completed manuscripts by a mathematical formula based on page count, rather than as determined by word processing software. If you use an old Underwood typewriter, I suppose his way makes sense, but it seems defining a broader word count target for a genre novel would be more appropriate than eliminating "a half page of space" at a time by combining two chapters into one. Maybe this will make better sense to me when my manuscript is done and I actually have a word count in hand ("No!" my self-doubt screams, "You've found the ultimate flaw in the plan! Don't attempt to follow it! Never begin work on your novel! Use more exclamation points!!!").
Ultimately, the plan boils down to this: know your characters ahead of time; map out your plot before you begin to write, basing each section on what your character(s) did in the preceding section(s); remember to take your characters' emotional states into consideration as you develop your story and reflect it in your writing; put three big escalating "surprises" in the story and drop them in at the appropriate places within the novel; keep your prose tight and your points-of-view focused. Do you have to wait until your second draft is complete before breaking the book into chapters? I don't know, but the basics of the plan are sound whether you choose to follow them to the letter or not.
