I still use my 5 Subject spiral from college for work and personal writing. I only filled up two of the subjects in it during my last year so it still has plenty of pages. It's kind of cool to in one book my writing from when I was school and my writing from the present day.
I've had several pocket notebooks throughout the past decade of my life. It's a habit that I've fallen out of in the past year, hopefully this thread will inspire me to rectify that.
I am very partial to Moleskines due to their perfect (back) pocket size, as well as their widespread availability. They're generally cheap - I think a pack of three is $12 or so. They're also fairly durable. My patronage of them outside of this is largely due to momentum and familiarity; I'm sure there are other great pocket notebooks.
If you haven't tried carrying one, I would highly recommend giving it a shot. They're super useful! My usecase was generally using it for any time that I would need to jot something down for myself or others - ie, making lists, taking notes, leaving notes for people, general scratchpad. In this respect, it was very helpful for keeping myself accountable to my responsibilities, as well as generally improving the quality of communication I had with people (externalized memory is helpful.) The other general purpose for it was just jotting down ideas that I had, as well as organizing other content.
Highly recommended practice. It also comes with the bonus habit of carrying a pen!
There are some inherent flaws in the format, like any. 1) Unless you approach the notebook with some sort of structure that helps you categorize content, it can be disorganized. Similarly, if the content is not regularly gone over, content put down in the notebook can remain as ephemeral as the thoughts they represent. 2) possibility of losing notebooks, or as happened to at least two of mine, liquid damage.
Pic related is most recent iteration; it's been languishing in disuse on my bookshelf for too long, unfortunately.