Before using Headway, I spent my commute by doing useless things like scrolling social networks but then I started using it and now every trip in the subway is an occasion to learn something new.
I specifically like the multi-days challenges because you can choose a topic that you want to focus on and start learning it with multiple short sessions.
I am really glad that I bought the subscription to Headway and I suggest it to anyone that wants to fill his daily 10/15 mins gaps.