I literally downloaded every single free app that came up under "Spanish" in the iTunes store. Duolingo was on my backlist, because it required registration immediately before you could use the app. Those apps I usually delete straight away. But since you say it's good, I went ahead and registered. Didn't get too far, since very quickly it required headphones to complete a task, and those are at home (it's impossible to hear anything on the subway).
It's one of the reasons I like it. I can listen to spoken speech and translate it and it also tests my spoken speech.