-Do you write down and keep everything in your brain, a la Getting Things Done style (write down notes and checklists for anything at all you would normally need to memorize)? Or is it more so used as a solely broad, project-focused mind map.
For me TheBrain is a multifaceted application. I use it both for projects and also for writing notes (I have hundreds of thoughts specifically for notes) that are interlinked both via regular thought links and also links to thoughts within notes, as well as storing links, adding notes about sites, or files. Storing/reviewing books and movies I've read/watched. The list goes on...
I generally don't use it for GTD stuff, I find other programs better suited to that task. However, I do use lists in notes in TheBrain, especially for projects.
-Do you have just one main "Master Brain", or do you have several more specialized brains? How many brains do you have at any one time?
I have one Master Brain, but also have specialized brains. Some due to wanting to share information publicly, some because I need it focused to a specific area and separate from my regular data. Condensed information from my specialized brains usually make it back to the Master though.
(this has evolved btw, I used to only use 1 Brain).
-Is your brain more personal-use-only, or is it shared with anyone?
Master and specialized brains are personal use only. One is specifically public.
-Are your brains mainly work-focused, personal-life-focused, or used all round?
They store both work, personal life and everything in-between.
-How often would you say you open, look at, or make edits/additions to your brain on an average day?
My master brain is always open. It depends. New projects I'm editing all the time. I'm always referencing though, so anywhere between 10 to 100+ per day, depending on what I'm doing.
-What would you say is your most used functions in TheBrain? The least?
Most used -> Timeline, Normal view, outline view, reports, search, linking, adding attachments and viewing.
Least used -> Extended thoughts view (which is fortunate since it's not in v.9 yet).
-How specific/extensive are your "thoughts" and notes on average? (A blurb? A sentence or two? A paragraph? A full blown essay?)
They are both specific and extensive. Usually anywhere from 1 sentence to 3 pages worth. Any more than that and it goes into a text document as an attachment.
-How big are your brains on average? Which is the largest? The smallest?
My current Master Brain is new and was started a few months after v.9 alpha was released. I ended up transferring some of the information from the old master to the new one, it's now an archive. Current size is ~4,000 thoughts, 1,287 notes, 592 files, ~2300 attachments. It's very information dense.
Smallest brain is my test brain with less than 100 thoughts. Others range from 100 - 500.
-Is the brain your only/main note-taking app? If not, what others do you use? Why do you use them instead?
It's slowly becoming so. I also use Ulysses because it's on both macOS and iOS. TheBrain 9 has been macOS only until very recently. As the notes editor has been slowly improved and the bugs ironed out, I've been using it for text entry more and more. Now > 60% of the time.
Anything written elsewhere will end up in TheBrain notes if it's useful.
-If you were me, would you have had all of what I've written and asked for already under a "questions to ask /u/theeagleandme and other TheBrain users" thought?
Wow, that's a fantastic question! I'd probably ask for ideas on how it can be used and how people use it. This is old, but still good and there's lots of information there you might find useful.