I’ve used it but just did whatever they said to do next.
I used to have a large number of Vietnamese high school students regularly corresponding with me to get me to help with their English studies. After a few months of essentially proofing and editing their classwork I encountered Duolingo. I tried it with Vietnamese and thought it was very well constructed. It must be good. I sent the link to the students and don’t hear from any of them any more about English class work. I have my goddaughter using it and she reports that she has gone from being a pretty good English pupil to being way ahead of the class. My own granddaughters use it for French and Spanish. Amelie, now 12, who is doing the French has added the Vietnamese to her efforts because I have promised to take her with me to Viet Nam in two years.