A few weeks ago I filled the 1000th page in my Bullet Journal. Actually, I don't think I can call it that. It's not in fact that much oriented around bullet points. It's just a series of notebooks with consistently numbered, dated and titled pages for easy referencing, monthly indexes and easy-to-see square bullet points for denoting tasks. Most of the things I said two years ago still hold, so I'll try not to repeat myself too much here.
Almost everything I do these days goes into this journal. Lab notes, sketches of ideas, random thoughts, doodles of happy ponies, to-do lists, pages worth of crossed-out mathematical derivations, interesting quotes, meeting notes and so on. Writing things down in a notebook often significantly clears them up. Once I have a concise and articulated definition of a problem, the solution usually isn't far away. Pen and paper helps me keep focus at talks and meetings, much like an open laptop does the opposite.
Going back through past notes gives a good perspective on how much new ideas depend on the context and mindset that created them. An idea for some random side-project that seems interesting and fun at first invariably looks much less shiny and worthy of attention after reading through the written version a few days or weeks later. I can't decide though whether it's better to leave such a thing on paper or hack together some half-baked prototype before the initial enthusiasm fades away.
The number of pages I write per month appears to be increasing. That might be because I settled on using cheap school notebooks. I find that I'm much more relaxed scribbling into a 50 cent notebook than ruining a 20€ Moleskine. Leaving lots of whitespace is wasteful, but helps a lot with readability and later corrections. Yes, whipping out a colorful children's notebook at an important meeting doesn't look very professional. Then again, most people at such meetings are too busy typing emails into their laptops to notice.
As much as it might look like a waste of time, I grew to like the monthly ritual of making an index page. I like the sense of achievement it gives me when I look back at what I've accomplished the previous month. It's also an opportunity for reflection. If the index gets hard to put all on one page, that's a good sign that the previous month was all too fragmented and that too many things wanted to happen at once.
The physical nature of the journal means that I can't carry the whole history with me at all times. That is also sometimes a problem. It is an unfortunate feature of my line of work that it is not uncommon for people to want to have unannounced meetings about a topic that was last discussed half a year ago. On the other hand, saying that I don't have my notes on me at that moment does present an honest excuse.
Indexes help, but finding things can be problematic. Then again, digital content (that's not publicly on the web) often isn't much better. I commonly find myself frustratingly searching for some piece of code or a document I know exists somewhere on my hard disk but can't remember any exact keyword that would help me find it. I considered making a digital version of monthly indexes at one point. I don't think it would be worth the effort and it would destroy some of the off-line quality of the notebook.
As I mentioned previously, gratuitous cross-referencing between notebook pages, IPython notebooks and other things does help. I tend not to copy tasks between pages, like in the original Bullet Journal idea. For projects that are primarily electronics related though, I'm used to keeping a separate folder with all the calculations and schematics, a habit I picked up long ago. There are not many such projects these days, but I did on one occasion photocopy pages from the notebook. I admit that made me feel absolutely archaic.