I'm sure it's not the best language learning tool around, but it has taught me enough of three different languages to visit a foreign country, ask for directions, order things, etc, with a number of conversations never falling back into English. And all for the price of free.
It all depends on one's expectations. I have no expectation of speaking a new language correctly or with a decent accent (I'm probably too old). I've being using Duolingo for some time and I complement it with Youtube videos and random texts. My French spelling has improved and I can read Russian news. I'd call it a success. Of course it won't replace a proper course and teacher, but it's not worthless.
