Read the book and finished! I love her style, and I think I'd love them in person. They just seem so....NICE (in the best way, not the *meh* way.) They have a fun story, good writing style, and what looks like a happy marriage and family. All goodness there.
Many of the Amazon reviewers got one thing right - these two have it made due to hard work and a lot of teamwork. They also note that: 1 the frugalwoods "are not retired." and 2 the frugalwoods "aren't middle class."
I will say that both of these points are *somewhat* fair. I'm not the retirement police - but these guys do have a full-time, 200k+ job working to their benefit at the moment. They are FI, according to their (undisclosed) definition. Hence (with Nate's job), the second point: they are/were in no way "working for lowly non-profits at modest wages."
They did start out with zero income or whatnot, but it sounds like by the ripe, very elderly age of 25 (/s), they had the income thing on lock-down. This is actually very impressive, but not exactly spelled out in the book. The book gets very fuzzy around the time it appears their wages (and lifestyle) balloon suddenly with very little explanation. I feel like Liz and Nate are going to be alarmed that people had the wherewithal to pull up Nate's salary (public record) and accuse them of "lying" - it's going to hurt and I think they may not have expected that with the publishing of the book. Not saying they shouldn't have, but they may not like it much....
They also built this arc in the book that they're VERY loose with money then over a single cup of coffee, they find full, unrelenting salvation in frugality and are retired within 2 years. I know books need arcs...but: they very likely saved 40-50% of their (again, large) salaries for each of the preceding 10 years, which really does add up to most of the FIRE journey. This isn't exactly spelled out in the book, which may lead non-MMM readers to assume that just buying no clothes for a year or three = instant retirement.
I have to add a HUGE caveat that: they really are my soft-spot FIRE folks, and I do feel like what they're doing is unique and still very very impressive. I also identify with them (part of a 20's, DINK couple with a 300k+ pre-tax income) - the pressure to buy shit you don't need gets bigger, not smaller, with that kind of cash. But, saving 50+% of their income is, I will be honest: not hard. Just don't buy the Range Rover and you're good - many of the lovely readers here who I lurk on have much more skill, dedication, and mandated frugality in order to achieve the same dream as the frugalwoods because of more middle-class income levels. You experts, unless you especially like this pair as people, may not learn much from the book.