This gets 5 stars because it does what its designed to do incredibly well. Its super easy to pull out the app while waiting in line or something and chip away a little more at your target language.
With the said, it cant be your only tool in your arsenal, especially since things like grammar are implied in its various scenarios, but not explicitly explained.
This is NOT a problem with the app. This statement is more about realistic expectations.
Hanzhe about Duolingo Learn Many Languages for Free, v5.0.15