It will be a diet - or a lifestyle change - depending on what one makes of it. ANY weight loss plan is temporary and will lead to a maintenance WOE (way of eating) OR people will revert back to what made them fat in the first place. You can't blame the diet, if someone blows their own maintenance.
The HCG diet is extreme, but it's full of healthy, fresh whole foods. No processed crap, no sugars, and built in portion control. Those are good things that everyone who is trying to lose should practice. I did a round of hcg dieting last winter and lost 33 lbs. I've kept it off. The reason I've kept it off is I know I cannot eat the way I used to, and I've retrained my body to enjoy different kinds of foods. My snack of choice now, is fresh strawberries, or stuffed celery.
I've made a conscious decision to live differently. My motto is eat like your great grandma did, if you could have found it on your great grandparents table then it's OK to eat. If you can make it from scratch like your great grandma did, then it's OK to eat too.... moderation called for when needed. If I want pasta or a cookie, then I must be prepared to spend the time to make them from scratch, so I'm not eating that kind of stuff every day.