First I have not used the app but IIRC it uses a lot of "Choose the correct word among those four" but it's quite bad for reinforcement. It's much better to reinforce what you learned to have to type a sentence from scratch. I recommend using it on a computer. Also, I don't know if that exercise is still ther, but having to pronounce a sentence is also good. The problem was it was too easy to pass though.
The other thing is that it does not replace learning the language, that is reading a grammar in order to know what are the general rules of the language. But it does replace conversation if you are not able to speak the language and provide a a good source of daily reinforcement, and vocabulary.
It's an effective tool when used correctly in my opinion.