(I give this app a 5 because it is excellent, however it is broken and until it is fixed, it should get a 0. )
This is the best designed system of education that I've seen short of the Khan Academy. I truly believe that full curriculums will look something like this. That is why it is so sad that they have not fixed the problem it has. Suddenly after updating, it stopped working for everyone and they have not fixed it.
It is possible to get around the problem by turning off wifi every time you use it. This disables some of the advanced features (charts for parents), but the main app works fine. But we shouldn't need to do that. They should fix the app.
It does work on my new iPad Mini, but does not work on my iPad. From the long list of people on the iTunes page, it looks like it is broken for everyone else too.