Don’t you think the whole market of language learning apps should be looked at with suspicion? I talked to my brother-in-law the other day, and he said he hired a tutor in the Philippines to teach his kids English. Unsurprisingly, it was super cheap. Low paid labor might be hard to get for Japanese, but it seems like for most languages people want to learn lessons over Skype will be far greater value than even a well designed app.
I think it depends on what exact level you're currently at in your language learning.
If you're a total beginner then I personally think that having a teacher who can actually explain things is invaluable. If you don't have the basic rules of the language internalized and you don't have the fundamentals down you're going to have a harder time doing it on your own than someone who can just ask a teacher and immediately get an answer.
I think the sweet-spot for language learning apps (at least for Japanese) is learning vocabulary and kanji. Those are two things where having a teacher doesn't really help all that much, and where an app can really help.
But, again, a total beginner (which is the market that Duolingo targets since that's where the money is) doesn't need to grind vocabulary nor kanji; they need the fundamentals, which they won't really get from it.
