I have kept a journal for a couple of years now. I wish I was organized enough to look back at it and be able to look at what helped / what didnt', etc - most of that I do by memory. I"ve worked w/ a couple trainers in the past who would track the weights / reps, etc but then they'd leave the gym or whatever and then I have no record. I worked w/ one guy about 10 yrs ago who had me doing some stuff that gave me the best results ever and I just can't remember what it was. I also have a 3 month hole in my 2000 training journal from the last trainer - I paid him a shit load of money and I have no record to go from now.
The biggest value I get out of it is as a general reference: seeing what types of exercises work the best for me, what splits I like the best, what things hurt, and different exercises to change to when I need to be creative. The other is that I can track my day-to-day improvements or impediments and also check what was my last workout weight level or rep leve so I can try to improve on it for the current workout. (I hate when I can't improve either the total weight or the total reps or something for every workout).
The food journal I started, but like VDL, I thrive on consistency - I eat the same stuff all the time, but what I do track is the different general changes I make. I.e. if is start carb rotating or make a big change in order of meals or something. I"ll write down the general formula for one day and then follow it w/ some variations. This is also useful for going back to - i.e. i had great results from the cutting diet I followed for my first comp, but I personally felt that the total cal content was a little low because I constantly felt weak and starved for the last month. So I would like to go back and try it again, but with a little more food. Or, since I'm using a new trainer now, I can refer back to show her what I have done in the past and i can also write down what she is giving me for a diet.
I use those small, but thick note books (the size of a small address book). Anything bigger and it will get crushed in my gym bag or I'll just get annoyed with it and not carry it around.