I’ve not used it for GTD, and I’m not sure how well it would work for this.
However, for knowledge management, I really liked it. I have a very hierchical approach to knowledge management, but find mind map apps like XMind quickly become overwhelmed as structures become complicated. PersonalBrain uses a mesh rather than tree and branch architecture, so can scale as far as you need.
It also creates a structure on disk that can scale to whatever size of KB you create, not just one huge file.
I used to use it for tracking meeting notes and relationships - for my industry, companies, people, organograms, and how those people related to others, products, skills, companies based on conversations: I then wrote all my meeting notes in it and linked to those people as attendees.
It meant I could pivot from a meeting, to an attendee, to all the other times we’d crossed paths (other meetings, mentions by others, etc). It revealed history and motives that I otherwise just wouldn’t have remembered.
The only problem was that it was a lot of work to update it with my notes after each meeting, and I didn’t want to trust the cloud to some of this data, but wanted to have it acccessible on my phone via the mobile app.
The mobile app is pretty good, but not fluidly native - they’ve tried to create all the functionality and features that people love in the desktop app, and it doesn’t quite fit.
But yeah, it’s a hidden gem. If you think and organise in this way, and are willing to go all-in, it’s incredibly powerful and seamlessa