It works. And it's written by someone who's (a) learned multiple languages himself, (b) done the research on human language learning and memory/expertise to condense an immensely effective method into a book.
He's gathered a vast amount of material for French, Spanish, Japanese, Chinese, and more on his website, https://fluent-forever.com. I've used his pronunciation trainers for Spanish and Japanese, and it's obvious the enormous effort he's put into all of his materials. Wyner obviously loves his work.
Learning a language has no shortcuts - only different methods with varying levels of effectiveness, i.e. fluency gained per hour of work put in. I can say that of all the apps and methods I've tried, from Pimsleur to Rosetta Stone to Duolingo and Memrise, Fluent Forever is by far the most effective one.
I used it to learn Spanish for a few weeks - his approach of learning pronunciation first, then basic vocabulary, seems counterintuitive at first, but it really, really works. Which I found out to my pleasure when I went to Spain on vacation with my friends. Even though they'd studied Spanish all throughout high school, both my pronunciation and my fluency were miles above theirs. I couldn't employ advanced tenses, but as he explains in the book, it'd be impossible for me to fluently internalize more advanced grammar before the simpler forms for any language, anyways.
I've moved on to Japanese for now, since I have a much longer trip there coming up. This will take much, much longer (it's a Category V language versus a Category I language), but so far it's been just as effective at teaching me pronunciation and hiragana/katakana (their phonetic lettering system).
I highly recommend the book - it's practically free at that price. Compare it to the $300 you'd spend on Rosetta Stone and don't hesitate.