Hi Shadow, I meant to respond this when I last saw it but somehow I did not. Sorry!
Conventional flash cards do great when it comes to practicing kana characters; that was the reason it remains the de facto standard on the subject. However the prerequisite of that is the player must have memorized the characters at least once before.
The main goal when I designed Stackana was to bridge this gap: players do not have to memorize any kana characters prior to practicing them, they practice and memorize as they go.
So to answer your question, yes, I have learnt a handful of kana characters as I played. But my day job and side project have definitely made me play Stackana less than I once hoped for. And that is the long-term goal of Stackana project: how to bring Stackana to players, instead of having them go to play it.
So stay tuned!
P/S: Is that you real picture in your avatar?