Then I started using TTMIK as my main curriculum and sometimes pull up Duo just like flashcards and that has been much more productive.

I used Duolingo exclusively for 6 months and feel like I learned very, very little. With each section of the 'tree' they will have cards that explain the more subtle points, but unless you go and specifically look for it, you will never see these. The other piece with Duolingo is that they do have the deeper grammar, pronunciation, and explanation points, they are just hard to find and the program never makes you learn them. Duolingo is a supplement at TOPIK Level 1 at best.

The social/gamification elements are also really positive - they provide constant measurements of success, especially with language learning being one where you sometimes are just in the doldrums and not feeling the progress. So obviously there are better resources out there and better ways to spend your time learning korean, BUT there is a huge BUT: Why you still can use Duolingo. You can jump on and in 5-10 minutes review vocab and keep that part of your brain active. Duolingo is great for quick refreshers or for making sure you keep Korean 'top of mind' if you live in a country that doesn't use Korean.

Duolingo learn korean from english