How I Use It
I am a high school Spanish teacher. I use this for occasional enrichment in all levels of my classes. For instance, if a quiz takes 3/4 of a period, I might allow students to use DuoLingo for the remaining time. It reinforces skills and gives students a preview of things to come in our classes. I have some students retain enough so that when learning about new concepts or new vocabulary in class, they can recall having some exposure from the app. I have some students who immediately click off of the instructional text that appears once in awhile. I also have some students who are not that eager to use this the more we do it. That is why we really only use it once in awhile. DuoLingo is set up well. The content is useful, and it gives students a sense of achievement by allowing them to reach goals.
I like this tool in moderation. It is definitely not a substitute for a language class, however. It is obviously limited in scope, yet, as I mentioned, I think it encourages students and gives them plenty of exposure in ways that they will remember. It incorporates visuals, has six languages available (as of now), and it uses all four language skills (reading, writing, speaking, listening). In the Spanish program, it gives users examples in accents from different countries which is a great feature. I also like that if you don't use it for awhile, I requires you to back track a bit to "regain strength". Last, I think it is helpful that advanced learners and native Spanish-speakers can take a "placement test" once they create a profile so that their weakest skills are targeted right from the start.