The surgery controls your appetite which in turn keeps you from overeating, at least in the beginning. During the first 18 months or so your body and attitude towards food adjusts to the healthy eating habits you have learned. Those of us who have managed to lose weight and can't manage to keep it off need the "extra" help of the surgery to be successful for the rest of our lives. Even with the surgery, permanent weight loss requires due deligence and is not guaranteed unless you follow a healthy eating plan. For me, sticking to a weight loss diet alone is just not enough and the surgery will give me the edge I need for a successful and permanent weight loss. To answer your other question, staying long term on a diet shouldn't be harmful if you eat sensibly. Seeing your doctor and a nutritionist while dieting, with or without surgery, will help keep you healthy.
I JUST WANT TO MENTION TO EVERYONE THAT KEVIN HAS MADE US A NICE CHAT ROOM. PLEASE STOP IN THE CHAT ROOM WHEN YOU VISIT THE FORUMS SO WE CAN ALL GET TO KNOW EACH OTHER BETTER.