Personally, I prefer apps with complete sentences and notes about everything. 

Apps like Duolingo are very basic, they repeat the same few words without giving any really hints on how to use them.

I've seen lots of spelling mistakes, as well.

I found Duolingo to be a great starting point to learning Spanish.

I´m not sure if I´d still be learning Spanish if I hadn´t found Duolingo. The classes are short which really helped me to get into a habit of learning at least a little bit every single day. Today, I have more or less an intermediate level in Spanish.

That being said, I did reach a point where I had to find other platforms (i.e. italki) to continue learning.

I think Duolingo is very useful for beginners as it focuses on vocabulary, rather than grammatical explanations.

Purak,

If you have an advanced level in English it is useless. It is like to go to kindergarten after university. Or you think to start learn another language?