Duolingo is great, I use it too and know people who got results with it. I don't think it particularly shines on general vocabulary acquiring though¹, so that's where Memrise or a spaced repetition flashcard system like Anki or any other enters the picture.
¹Actually, this is sort of covered by the translation exercises, but since there is not a system to manage the reviewing of new words you learn there, I think it's inferior to a flashcard system.
I really want to use Duolingo, unfortunately I am learning Mandarin, and it is not available yet. It has been announced, a while ago, that it was in their todo list, but I don't know when it will actually be there. It is not even in the beta language section yet.
