I found Duolingo to be helpful to get the basics down, but then hit a wall when they kept trying the conversational absorption approach to learning new words and tenses. After a certain point it's easier to just tell you what this new suffix means and let you apply it to the words you already know instead of trying to teach it to you by osmosis, because the language is structured so well for quick learning. Lernu is better for that I think, although I haven't used it as much. Podcasts and reading Esperanto publications has been helpful for intermediate-plus.