So I had a look, liked its gamified way of teaching and signed up along with several friends. I was having so much fun I added German which I had learned many, many years ago and most of which was long forgotten - and, working on the assumption that it's better to take things slowly, I'm still going and doing at least one lesson a day in both languages although all my friends have long since dropped out and the travel never eventuated.
It's not the perfect language learning system but it does what it sets out to. Since it uses the immersion technique there is minimal grammar teaching. The idea is that you pick it up as you practise and to a degree that does happen. There's also the option to comment where questions can be asked and are answered by other students and the moderators. Even so I find I need to refer to other sources and there are many on the internet. I know! Who'd have thought it!
Because when you answer a query you are expected to provide a reference link or explanation, there's been an interesting and unexpected side effect. My knowledge of English grammar has improved enormously. All those things that I just knew have to be justified and while my grammar has always been pretty good I now have a much better idea of why something is said or used in a particular way. It's been very informative and, most surprising of all, is that the best sources of grammar information are websites where English is taught as a second language.
Am I fluent in either language? No. Will I be just using Duolingo to become fluent? Probably not because you need to speak a language to become fluent and that means interacting in conversations, not just parroting back what you hear. Does that mean it's a waste of time? No. I already have a reasonable vocabulary - enough to read quite a lot - and if I keep on - and find myself a conversation group (the next important step I think) I should soon be able to hold at least a basic conversation.
Wish me luck.