I wouldn't call it "pretty useless". I think it has two distinct places in a language learning process.
1) They make learning a new language very accessible and fun to get started. Books and courses do not have such a low level of entry.
2) It's good for learning new words in a context. Memrise for example often just gives you a single word and translation. But I think it's much more effective to then put words into sentences.
That said, if you really want to learn a language past the "I can manage for a few days as a tourist" you need to add other methods to it. Read books, listen to audio content, watch TV, have lessons (speak the language!), study grammar etc.
Then not everybody is alike. So try different methods. See what works for you. Some might have more success with Duolingo, some will have less.
1) They make learning a new language very accessible and fun to get started. Books and courses do not have such a low level of entry. 2) It's good for learning new words in a context. Memrise for example often just gives you a single word and translation. But I think it's much more effective to then put words into sentences.
That said, if you really want to learn a language past the "I can manage for a few days as a tourist" you need to add other methods to it. Read books, listen to audio content, watch TV, have lessons (speak the language!), study grammar etc.
Then not everybody is alike. So try different methods. See what works for you. Some might have more success with Duolingo, some will have less.