Pros:
- Daily repetition can help build up memory and a bit of confidence in learning a language. However, the progress is like the walking speed of ants.
- There are people who are fluent in the language to answer your question and doubt (Only IF you are not banned from speaking on the discussion forum in the first place).
- Practice questions are plentiful and in various forms which helps in learning the language in depth.
- It's free but you actually end up spending more time watching ads and playing games than learning the language.
Cons:
- Duolingo actively sensors personal opinions. If you made any comment against their belief, you will be banned from posting any question/answer in learning the language without any prior warning or explanation of the reason why you were banned. So to speak, you will lose your right to speak in the "language discussion" forum indefinitely and you can't find anybody or anywhere to straighten things out. They act exactly like a dictator in a communist country.
- A word of caution to people who prefer traditional mentality, Duolingo is promoting the LGBT mentality to its core.
- Duolingo is for beginners in learning a language. It helps you get an idea of a language but you need to move on to more effective language teaching classes/books/websites... if you really want to be fluent in the language you are learning.