The bad: Some people report being weak and/or tired during the fasting period. Both of these will obviously have a bearing on your daily tasks.
The stupid: Some people still manage to turn this into a binge-fest. If you think fasting is a magic bullet to a diet exclusively of junk food, think again. Personally I’d be liable to kill all around me if I was asked to survive on 500cals, even if it were just for 2 days.