I plan on adding a community college class into the mix, so there is some education outside of the app that will happen eventually. But, there's at least a few reasons why you'd use Duolingo exclusively.

1. If you can't afford something better. I keep getting pummeled on the leaderboard by people doing 5000+ XP/week worth of English for Spanish Speakers. None of them have Super Duolingo (you can see if they have it by looking at their profile)

2. If you're testing the waters. I already talked about the money, but a semester of a language class is a big time commitment, too. You also lose scheduling flexibility that's especially desirable when you don't know if you want to commit to a class yet.

Then there's supplemental, extracurricular practice. If you never use a language outside of class, you'll lose it very quickly. Anyone who had to take a language class in middle/high school knows this. Duolingo (or any other app) can help reinforce learning, by regularly poking the "foreign language" part of your brain, even on days when you aren't in class.

The supplemental is what I'm using it as, in between my A1 and A2 in-class lessons I'm using it to keep my level up and improve my ability to sentence build. New vocabulary is also nice.

I'm definitely progressing. I won't get fluent, but my level is definitely higher than that of languages I learned in high school, and I can read signs and stuff. (In fact, I think the method is fairly similar.)

The reason I use Duolingo because it's the only thing I'll actually use, thanks to the emotional blackmail.