My friend Paul and I fell for Chinese while studying in Beijing and later in Suzhou. Like most learners who get serious, we ended up in Anki: building our own decks, fixing them, keeping them alive. It worked, but it took hours before the first card was ready. And reading, listening, speaking and writing never fit on a flashcard anyway.
So we built Gushi: the whole language in one app, with spaced repetition deciding what comes back and when.