It absolutely has. I don't think I can live without doing it anymore. I started in middle school and am currently in college. Being so close to me, it's not formal at all, being more like a stream of consciousness filled with anything I think is particularly interesting to myself. I don't do it according to any set schedule; just as I am not equally energetic and insightful every single day, I am not equally productive every day.
If I don't write something down, I forget it and move on to the next idea. While I rarely read what I have written, it is a massive benefit by itself to commit to paper what I am thinking because the process of transforming a pool of unorganized, but related thoughts into something resembling a cohesive idea that can be told linearly is invaluable in resolving contradictions and evaluating their value. I prefer to handwrite because I don't think in ASCII (or Unicode, or LaTeX, or any other computer format). Ninety-nine cent composition notebooks and commodity rollerball pens are all I need.