Duolingo is okay, especially considering it's a free app with no setup, but it's not perfect. It's lacking in explanations if that's something you're looking for those, and sometimes it's too particular in how it interprets things (大変 is always "very"). A few more apps that you might be interested in:
Tae Kim's Guide. It's a textbook app that focus entirely on grammar. It's great at what it does, but specifically what it does; it doesn't have exercises or things like that. Looks like it's not on the Play Store anymore, so you might have to find an APK? I think it was a free app.
AnkiWeb. It's a highly customizable flashcard program. You can download premade decks, but you get the most out of it if you make your own.
Human Japanese: These are ones I enjoyed using, don't think a lot of people in this thread went this route. It's a set of textbooks that focus on readability; they also have a variety of tests you can do at the end of chapters or as review. The main problem I'd say they have is that they ease you in a bit much, so it takes longer than it has to. There's a free trial version of the beginner one if you want to check it out, but you have to pay for the full version and the second textbook.