Great! すごい!
I love this. Thought it might just be kind of a toy, but I actually really like the natural-looking brush strokes. It takes a little while to get used to using fast or slow movements to vary the thickness of your brush strokes (with real brush calligraphy, this is done with pressure), but the results look quite good with a little bit of practise.
Two things I wish this had: multiple undo/redo (it will only undo or redo the last action), and a built-in gallery for your creations instead of having to save them to the camera roll. All the other art apps I use have their own saved files; this is the only one that makes you export them to JPEGs that get mixed in with your photos. Other than keeping them separate from your photos, the advantage of storing them within the app, in the native document format, is that you could change the style (background and ink color) of previously-made art. This is possible when actually working within the app, but once you export to JPEG, that is, of course, impossible. Maybe thats the "Zen" of it though (Zen emphasizes trusting your instinct and not second-guessing your actions, so perhaps its appropriate).
If you have any interest in Japanese/Chinese calligraphy, definitely give this a try!
Stormchild about
Zen Brush, v1.15