It's only since last few months that I have kept a seperate writing journal. As I prepare for NaNo, attempting to focus on only one project, it has become a journal more for that particular book, though I still have a few odd ideas and things written down in there.
This experiment of keeping a writing journal has been quite rewarding. Usually, I tend to scribble notes all over the place. I try to keep a specific notepad for a book, but I always end up writing in all sorts of places, and obviously don't have my notepads at all times. And I seem to accumulate so many notes (because I sort of think by writing) that consolidating them is usually a nightmare. With the writing journal, I have been able to keep everything together that's hand-written. There are still typed random notes in computer, but that's a different problem altogether.
One advantage of writing down the notes, and actually writing by hand is that I remember those things better. That's very handy when you are writing a fantasy novel, and have got a whole world to not only construct but also to remember all the tiny details. Also, my ideas evolve and change over-time, so again it's handy to have dated notes in a journal, because then I can make sure that I am using my latest ideas and decisions.
The writing journal experiment has definitely been a success, so I will continue to keep them. And another lovely thing about it that writing journals tend to take on a personality of their own, an extension of me, and by the time pages are getting full, you can clearly see it as you flip through the pages. So I get to see the workings of my own brain ;)