Very useful for simple math
The app has a sound design and beautiful aesthetics. The method learning portion could use some work - like a counter somewhere with the answers correct/answers wrong, and the icons for switching towards practice could be a bit more obvious, but that doesnt really take away from it.
The practice mode that includes all methods could indeed have an option to remove certain methods that are more complex - such as squaring numbers in the 400s - so the user could practice several methods that hes learned at once.
As for the instructions, I think they are pretty clear and if read with enough attention, are simple to understand.
Overall its a pretty amazing app and it has lots of potential for math-loving people. I enjoy it quite a lot.
Sakeuon about
Mencal - Mental Math Tricks