I encountered the BulletJournal method in 2016, and originally, I resisted it a bit, despite my curiosity... I have tried so many methods to stay on top of my tasks and feel like my life isn't spinning out of control on a regular basis. Some worked ok, some frustrated me thoroughly, some were just too high-maintenance (looking at GTD here), and having my productivity system fall apart was a pretty regular experience.
I'm glad I yielded to that curiosity. Like Carroll, I have ADHD, and somehow my brain responds better to a handwriting-(on-paper-)based approach than to digital tools. I also need an approach that provides structure and clarity, while being flexible enough to adapt to my needs and easy enough to keep it up as a daily habit.
The Bullet Journal method is a cross-over between a to-do list, a journal, project planning and, if you want to, even a calendar, kept with a system of symbols and shorthands that Ryder Carroll calls "rapid logging". The only things you need are a notebook (any notebook you want; I started with a very cheap notebook) and a pen.
My daily planning session now takes no more than 5-10 minutes (as a rule) and has become a cherished start into my working day. While I slack off on some days - mostly weekends or vacation -, the system as a whole has not "fallen apart" for me once since I adopted it as my one self-management method. I use my Bullet Journal for ToDos, project planning, logging exercise, tracking habits and sometimes for journaling (something I rarely did before last year).
So, I was a pretty regular user of the BuJo before "The Book" was published; I hesitated to read it and wasn't sure if I would learn anything new or useful from it.
And you don't need to read the book if you just want to use the method; there are good explanations of the core method on Ryder Carroll's homepage as well as on Youtube. However, I found a treasure trove of information, hacks, ideas, and how-tos in this book that helped me deepen my practice and get even more use out of the method. I keep going back to the book to review Ryder's suggestions as they become relevant to my needs in terms of planning and organization.
One caveat: Because the book encouraged me to do so much more with my journal, my BuJo has become a bit messier than it used to be before, and I now go through 2 or 3 notebooks per year instead of one.