Well, isn't using Duolingo a kind of self-study? Which languages have you learnt with it and to which level? I finished the trees for Spanish and Greek but didn't like it and also don't think that I got over A1 with it. It's good for killing some time in-between but not as the major resource for studying a language.

I think it is great app, but for me it is quite time consuming if you need to have quick progress. If you have free time (waiting somewhere, having a long trip or way to/from work...) it helps. As always for some people it works,  for the others it doesn't.

I think Duolingo is a nice app. I studied Romanian with it and I learned a lot of words. When I studied with Duolingo I had a notebook next to me and I wrote all the new words and sentences. When I tried to use Duolingo without my notebook like on the subway or while I was waiting for a friend outside, it was a waste of time because I passed the lessons without really memorizing anything. After I completed the course, I started listening to Romanian songs and reading some stuff. Now I feel that I need Romanian teacher to help me get to a more advanced level. I think it's a good way to get familiar with a language.

I tried this application, but I didn't find it can be useful for me.