Is Duolingo any good to learn Russian?

I've been using Duolingo to learn Russian for 9 days. I'm almost at the end of the first section. It's a lot of fun and I'm practicing in daily life, naming objects and trying to form simple sentences in Russian. I can read some things said online and in captions on videos, so I I feel that I am learning much more than maybe I would have done with a book. However my friends insist that Duolingo won't teach me anything.

Does this just mean that Duolingo is only good for beginners, or does it mean that Duolingo is teaching me incorrectly?