This is not a discussion of the Assimil method "per se" and of the "with ease" claim.
In these informal notes I summarize my personal experience with "Japanese With Ease" and express my personal fallible opinions.
I write about a specific student, me, trying to learn a specific language, Japanese.
I have always been bad at languages, have very little memory, have little time to study, am no longer young.
Japanese is probably a very peculiar language for western people like me.
Few weeks ago I restarted studying Japanese. I had stopped a couple of years before. I restarted from the very beginning using the very same textbook: "Japanese with ease".
In the past I had showed "Japanese with ease" to some Japanese friends, they said that the language taught was very good, correct but also "real", the language spoken by Japanese people in real life.
I had taken the habit of asking those questions after a Japanese friend had a look at a university textbook I had bought and told me that the Japanese in that book was spoken by nobody in Japan.
So, the language is good, the format of the book is nice and practical, and now that I study it again as "false beginner" the study flows easy and pleasant. Sometimes the humour that is present in most lessons is even really fun.
If I had forgotten my previous experience and did not take into account that I am a "false" beginner I would say: "Japanese with ease" is great."Use it my boy, use it , use it, it's a great method and, by the way, it is not true that Japanese is difficult ... the language of easy mysteries you know ...".
In my country, Italy, plenty of people say that compulsory military service was a nice experience. They say it years after having completed it, they happily forget that while they were there they nearly got a nervous breakdown, that they were counting the days to the end Etc.
When I started studying with "Japanese with Ease" I was a real beginner. I had no previous knowledge of Japanese, used no other textbook, had no teacher, no help whatsoever. I found the method to be extremely hard: a lot of effort, minimal understanding, leraning next to null.
Probably that was due to the fact that I am very bad at languages, but I think that the difference between my mother tongue and and Japanese (rather "big") had a major role in my failure.
The assimil method provides explanations to help understand the text but it is mainly based on the fact that the student "discovers" the language himself. The difference between my mother tongue (Italian) and Japanese made very hard for me to understand the lessons properly.
The book provides both a literal and a semantic translation of the Japanese text, but still there were many words or pieces of sentence whose meaning and role I could not understand.
Probably I am too drastic and utterly wrong but I think that for bad learners like me studying in a situation similar to mine (total beginners, self taught) "Japanese with Ease" is not very effective and not at all "with ease" if used as the only learning tool.
It is probably very useful and effective for other students and situations, in particular if one has already some knowledge of Japanese or if one has a teacher or someone that can help him when he is not able to infer a language structure.
Obviously the difference between your mother tongue and Japanese has a key role. I am told that Korean language structure is similar to Japnese even though the phonetic is very different. If that is true I expect the Korean version "Japanese with ease" textbook, if it exists, to be much more useful to Korean students than the Italian, French and Spanish versions are for students of these countries. (Italian, French and Spanish are very similar, so all have a big "difference" with Japanese, and that makes inductive learning hard).
So, should you use "Japanese with Ease"?
If you are Korean :-) if you are not a bad learner, or if you have someone that can help you, or if you also use other "standard" textbooks I would surely recommend it.
If you're a beginner, a bad learner like me, with no other learning tool or help, definitely not.
Get first a more "standard" textbook, a teacher, a classroom course and spare yourself stress and frustration.
After you've got these also get "Japanese with Ease", it will be a very nice and useful complement.