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, 45 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently 111.