I am a big fan of practice journals, and think any serious music student should have one. As a professional musician I have a ton of music that I have to work on at all times to be ready for the various rehearsals and performances I have going on. My practice journal helps me stay organized and avoid getting overwhelmed.
On top of the organizational aspect, which is enough reason by itself, there's also a ton of scientific research coming out these days that shows a real impact on learning from reflecting on what you've just learned. In music practice, this means let's say I practice a passage from a piece, then take just a few moments to think about what I did, what I figured out, what worked, what didn't, etc. and write that in my journal. Just the act of thinking about things in this way helps your retention of new information. Writing it down helps even more. Then if you look at your journal the next time you practice, you get another boost to your retention.
I've actually gotten so enamoured with practice journals that I've created my own online journal system, which I call my Structured Practice Method. I use it myself, and with all my college level students. It's really made a big difference for me.
You can check it out at www.drfleggspm.com
I am beta testing it right now, so it's completely free until the end of the year. After beta-testing finishes I'll still have a free trial period so anyone can try it without having to spend money.