Luis von Ahn

I'm biased because I'm the co-founder and CEO of Duolingo :) But, I'd say it worked very well for me in learning Portuguese.

When I had done about half the course, I went to Brazil to give a talk about Duolingo (in English), and the first question I got afterwards was what language I was learning myself. I made the mistake of saying Portuguese, and they proceded to ask me all other questions in Portuguese. This was the first time I ever spoke it with a real human, and it was in front of an audience of more than 1k people! I was terrified. But in the end it worked out pretty well. I was able to understand 100% of what they asked, and answer most things in Portuguese (though I sounded very broken). To be fair, I'm very familiar with the usual questions people ask after one of my talks, but still...

So for me, it has helped me go from 0 to a very decent level of understanding (reading and listening), but I'm shaky at speaking, mainly because I'm afraid of sounding stupid. This is something we're actively working to improve.