I'm a huge fan of this approach -- it's a hands-down the simplest, easiest and most effective way to learn or improve on a language.
I've been using the Fluent Forever methodology for years, and it works—I just have trouble sticking with it because of the busy work involved in creating flash cards. I'm excited about this app, because it incorporates features I've dreamed about while slogging through the card creation process on my own.
I've been using the Fluent Forever method to learn Portuguese over the last few months and I'm up to over 2000 words in my vocabulary and have been loving the whole process for the first time. This app is goingn to make the method easier to follow and build into a habit. I cannot wait for this.