I have kept one since March 2nd, 2010. It's just a single line or two at the top of each day's entry of my standard dairy - the food and estimate kilojoule count, with the total listed in the margin alongside.
I reply on mine. Without it I'd have lost weight much slower, and probably ballooned up again because of my bad "food memory". In past times I could spend an evening grazing the cupboard and fridge and not notice I had just eaten a day's worth of food energy in biscuits and icecream! But never again!
It serves a number of purposes:
1) Ensures I don't "forget" I ate a meal or snacked and try to make it up with extras.
2) Keeps me from eating more than I should.
3) Helps me plan future meals (I do my own cooking).
4) A constant reminder how little we really need to eat to stay healthy.
5) A record of success and failure that can reveal self-deception.