I had taken some Japanese in high school, and French in college, and later done some Duolingo to try to brush up my French, but then abandoned it.
I recently picked it back up again (couple of months ago), and tried doing both Japanese and French. I find the French one much more helpful. The Japanese module is still in beta, and it shows; tapping on one word to look them up if you don't remember them doesn't work well in Japanese but it works fine in French.
And yeah, I found the Japanese curriculum pretty lacking, and the kanji really hard to learn in this format.
I did find an interesting way to practice my hiragana and katakana, though. I created a slide deck using Tinycards (Duolingo's flash-card app, I found it a lot easier to set up and use than Anki even if it's less powerful) in which I would have to write the kana using the handwriting input method for Japanese. I found that actually trying to write out the characters was way more helpful for remembering them then just clicking on the right one out of a list.
However, I couldn't figure out a good way to get Duolingo to give me a list of the kanji I was supposed to have learned by now to use that with kanji, and figured it would be too much of a pain to go back through all of the lessons, write all of that down and then create the flash cards manually from that.
I've quit with the Duolingo Japanese lessons by now, I think I want to wait until they're a little more fully baked, and even then I'll probably need some other outside resources to study along with.
I think Duolingo in its current form is a good way to stay practiced with a language and learn some vocabulary and grammar, but only for some of the languages. However, even for those languages, you will need some other resources to really develop fluency.