I would say it's all-or-nothing for the first few months. Your body needs to convert its metabolism from fat-burner to sugar-burner, and that won't happen if you do it half-ass. After you're adjusted, you can cheat a little 80/20. But you can never go "off" the diet totally or the ailments will come back.
5'0" female, 42 years old.
Started Primal October 31, 2011, at a skinny fat 111.5 lbs.
Current weight: 101.5. lbs and holding steady. Spring yardwork here we come!
Co-worker 1: Needs to lose ~50. Now he wants to start Mayo Clinic Diet. Yeesh. Give it up, man.
Co-worker 2: Needed to lose ~55. Lost 20 from stress. Started Primal in Sept, lost 20 more, but gained 10 back on a carb spree. We're working on it.