J has been a stable App Store product for 5 years and the 701 build is the 3rd released version. It has a special keyboard better suited to J programming requirements and lots of minor improvements.
J is a general purpose, high-level programming language. J is particularly strong in the mathematical, statistical, and logical analysis of data and visualization of your data is at your fingertips.
If you are new to J and want to be a J programmer, this is a good place to start. If you already know J, you probably think of it as fun rather than work. Now you can have mobile joy of J!