I have been using Duolingo to learn a multitude of languages. I have learned plenty of vocabulary.
But is it effective for learning to speak?
It is useful as a supplementary tool.
Pros of Duolingo:
Words and grammar structures exercises are well organized. They are repeated often enough to make you remember. You will have decent understanding of the language's grammar and have a basic set of vocabulary after a while. (providing you see real language examples outside Duolingo often enough)
Actually, controversial issue, but it keeps many learners motivated for a decent amount of time.
Sentences like "elephant drinks milk" are not expected to be used in your real life unless you are a zoologist.
It is not reasonable to try to learn languages just by one-sentence patterns. A lot of massive input of native speakers' using the target language for their real life situations is crucial if you want to get fluent in any language. Duolingo doesn't give you enough of this.
Overall, it will greatly help your language learning providing you don't use it as a standalone service.
It's very effective if you use it... Effectively.
This sentence seems a joke, but you really need a good method to learn efficiently with Duolingo. Passive learning gives limited results.
It depends on your project, your memory type, the language you are learning, the way you are using the site, your habits, and a lot of factor.
A person can use the site once a month, and will say: Oh this site is really inefficient.
Another person will use the site at least 5 minutes every day, and use other tools, and make huge progress.
I learnt several languages with this site, so I wouldn't say this tool is not efficient, but I almost never use it alone.
You have to find your own method.
Mine is to use browser extension to pronounce the words, extensions with translations and dictionaries, and to use the forum to ask questions about grammar each time I have a doubt. Replying other people questions is a great way to learn, because you are forced to thing and to make some researches, more searches than if the question was your question.
I try to use synonyms in the exercises. I use other sites, and other tools, a dictionary, Memrise, etc... A grammar book if I can. It's like going to school, you wouldn't attend the course and think you don't need a dictionary because you had a lesson.
If you don't use the forum of the sentence each time, or almost, Duolingo will be useless, as you wouldn't have the grammar point. But if you know how to use their sentence forum, it's pure gold.
Sentences like "Elephants drink milk" are indeed very good way to learn, there are several reasons for this, but it's not the place here to debate.
Learning Romanian and beginning to learn Korean.
Duolingo is helpful. I probably practice and study a minimal amount of time. I am able to understand Romanian when I visit. Enough practice and it is possible to apply word meaning to specific situations.
I find combining Duolingo with other resources helpful. "Being there" is a great help. As with any learning drawing on several sources is helpful.
Duolingo is a great project in a sense of crowdsourcing the learning, and more importantly, creating the content on any language, in reasonable near future, from the users themselves to the users in every single language on the Earth.
Might be you won't use examples as "elephant drinks milk", but you'll get the idea, at least, how the words relate to each other in the language you are learning.
Also, technical implementation of Duolingo app is providing - just mesmerizing! One of the best sides of it is to check any possible steps ahead of your learning progress and keep of them in your hands. Literally.
Taking challenge of doing 50 points each day - very effective way of keeping consistency and taking high bars along the way. Besides, the app is providing three sides of learning process: writing, speaking, and reading. App is perfect choice for beginners.
Duolingo isn't trying to be total answer to all your questions - it's just one of the possible ways to learn, and create materials, in the language you are interested in, with the help of global community.