Incredibly valuable for me. When exploring something new, I journal as I go. If a side task has to be parked, I can pick up again months later because I use it as a knowledgebase. Also, a quick scan for #gotcha in my notes reminds me of subtle bugs and workarounds in tools and processes that are easily forgotten. I also create quick reference charts for tools I want to become familiar. I hashtag every new note as if I'm making a Twitter post, so things are linked in a useful way. I usually have Obsidian open near the IDE.