Is Duolingo a waste of time for studying Japanese?
i think you should stick to it, but also look into other websites as this course is still in beta. If your experienced enough with Japanese you should be fine, if your not, I suggest you look into it more.
A lot of discussions on Duolingo have some information and websites people offer to others for help so I say you should take a look at them. Hope this helps!
In my experience so far, it is a useful course to do a refresh if you already have some prior study background in Japanese, or as part of a more comprehensive set of Japanese study materials; but it is probably not the best course as a stand-alone learning method for a complete beginner, especially if you are using the mobile version.
After studying it for about two years and coming to Duolingo and completing the tree. I would say that it is helpful in the sense of Vocabulary, Grammar, and putting the two together. If you read the Grammar notes and memorize the vocabulary you will get the most of it.
Where it really lacks is Speaking and Reading (Seeing as there is few Kanji). You will have to practice those with other mediums. I also advise using books such as "Genki" I&II, Tae Kim's "Guide to learning Japanese", Heisig's "Remembering the Kanji" I&II, and a book that helps explain the confusing little bits "Making Sense of Japanese". Once you have sufficient knowledge of basic Japanese I would recommend listening to Japanese Music, watching Japanese Youtubers, Use a medium to learn with natives (Such as a Italki or a smart phone app called "Busuu", etc.
But would I say that overall Duolingo is a waste of time? No, not at all. It just needs to be not the only thing you use to learn Japanese. It should be used as a review tool, vocabulary, and grammar practice.
I spent the first four months of this year in Japan trying to speak to natives and using Lingodeer on my phone and watching videos and doing general study from textbooks (Genki I) and the internet. I memorized the Hirigana and Katakana on my own and learned some basic stuff during that time. It seemed like I was mainly just learning specific phrases despite my best efforts to learn the grammer and make new phrases. However, almost immediately upon returning to the USA I started doing Duolingo (by computer and using microsoft IME to type all responses, NOT using the wordbank) and some of the Japanese people I met in Japan visited me here in the USA recently and I found it interesting how much my ability to understand what was said and form original sentences had improved. Some of the wackier sentences that Duolingo makes you translate do a lot more for me to understand the principles behind the grammar than I was able to achieve when studying from other sources. I'm aware that this app has a lot of translation errors, but the discussions help work a lot of those out. The number of false negatives are a bit annoying (where it rejects a correct answer because you used the Kanji instead of the Hirigana). I can't evaluate how well Duolingo works from starting from zero, but right now it is my main study method as it took me to the next level. I'm not fluent but it caused something to "click" for me. I really think that typing on the computer has made a big difference for me compared to the word bank method though.
I took some courses on youtube earlier this year and now when I found Duolingo I think it has been great to test what I learned so far. I have also learned a few new things although cementing it does take time. As many have said elsewhere, using multiple sources is a very good thing and Duolingo should be one of them. It can't replace going to an actual class but a little here and a little there... ;)