I've been keeping one for about 2 years, I still fill it in every day out of habit. Really easy and simple. It has helped me to define a rank order of activities and how long I can safely spend on each one (shopping, kitchen work, playing music, and walking are things I don't do at all any more, but can manage 10-20 minutes if I have to, sitting at computer 40 minutes once a day if I'm lucky). I have a list of about 17 activities ranked in order of energy consumption, although most of them are things I don't do any more. I have recorded every day with a headache for the last 2 years, and by looking at what I was doing on the previous day I have managed to make a few connections. Tracking how I work (about 15 hours) over the week with symptoms has helped me spread it out to avoid PEM too.
I still fill it in every day just out of habit, but the only number I really pay attention to is the overall symptom level for the day. I have internalised the rest, so for a few months at the beginning it was really useful to find out what my limits were. I also somewhat foolishly hoped I would be able to come up with some kind of algorithm if I tracked enough variables for long enough, but that never happened. It's nice to see how the number of days lost to headaches per month has decreased.