I use this app on MacBook, iPad, and iPhone. Whatever I enter on one device is conveniently synchronised onto the others via Dropbox. Other cloud systems are available.
I find it very useful for authoring in connection with Scrivener. It is useful for building a stock of related ideas such as: characters, settings, research, or as an initial form of corkboard (chapter summaries).
It also works well as a form of mind map, sharing across all devices. A facility not yet available on Scapple. To use as a form of mind map set up an initial Book as an hierarchical root. Within this book create other Books or Documents for branches. Within a document you can create nested lists, either numbered or bulleted. I.e. You can indent a new list from say an existing bullet level. The beauty is that you can subsequently go in part way down a bullet list and add a new bullet point - which could itself then be indented for a new sub list. Or within an existing Book you can add further ones. And all of this is can be shared across your devices.
The developer is very helpful with advice and there is a useful forum and introductory notes.
Wonderful. Well done.

An Excellent App