It didn't work for me. I spent ~2 years trying to learn German and all I managed to retain are a bunch of nouns, verbs and adjectives. I am still unable to speak properly, let alone read a newspaper article. I suspect that the Duolingo "teaching style" does not work for me. It also depends on the objectives, since I was more interested in some kind of fluency as opposed to being able to read or write in German.