It's not especifically made for GTD but I find it perfect for it because of how easy it is to add items and because I value simplicity.
Any given line can be a project when it has tasks as children, and tasks can have notes as children. Any of these things are added instantly without selecting dates or priorities or contexts like GTD apps.
I move projects to the top of the list and/or color them red and/or open to see its tasks when I want them to show high priority (this is really easy with all the keyboard commands)
I color tasks blue if their context is outside (fortunately I live on just about three contexts)
The shortcuts window is really great as a "trainning mode"

The only downside as a GTD app is having to save the file instead of automatically saving and defaulting to the previously opened list(s) when the app opens again.

Also, a delay before opening parents when dragging lines up and down would be appreciated.