If you don't know much about the HCG diet, other than that a lot of people are trying it and losing a lot of weight on it, I'll give you a brief background (in my own words).
HCG is the hormone released by the placenta of a pregnant woman. When on the HCG diet, the patient either injects or absorbs sublingually (under the tongue) tiny amounts of this natural hormone. The doctor who started this diet observed that his obesity patients who used the hormone and followed a strict diet of lean meats, vegetables, fruits, and a very small amount of specific starches, lost--and kept off--large amounts of weight. The HCG tells the body to draw fat from the abnormal fat reserves (those on the body which enlarge fastest as one gains weight--often the tummy, hips, thighs, buns, etc.) rather than from muscle, healthy fat reserves (such as those that cushion the organs) or from normal fat reserves (those which can be broken down quickly to give the body energy when there is insufficient nutrition). Therefore, a patient on the HCG diet doesn't lose muscle or healthy fat (this would be counter-productive to a pregnant woman trying to grow a fetus), but from all the "right" spots. And as the abnormal fat reserves are unlocked and broken down, the energy from them is released into the blood stream, so that although the person is on a very low calorie diet, their body is actually recieving quite a lot of calories and shouldn't feel hungry. Also, the body begins to reset the hypothalamus, so that when the diet is finished and the maintanance portion of the diet is finished (3 weeks of slowly re-introducing normal fat and sugars), the person's metabolism is better and they are less likely to regain the weight than with any other redued calorie diet.
So that's the background. If you want to read the actual manuscript, it's quite intesting, and the whole thing is online here.
Most people lose 1/2 lb to 1 lb (more for some men) of fat per day. PER DAY. You can see why this is such an attractive diet. When I lost 18 lbs over four months on Weight Watchers, and tend to lose 1/2 lb per week when I exercise and try to eat right on my own (if I'm lucky), the idea of losing all the weight I need, which is about 13 lbs, in only two weeks of dieting (although you have to finish out the whole 3 week diet + 3 week maintanance diet if you want to keep it off) is absolutely amazing. I can honestly say I am EXCITED to be donig this diet. Knowing it will work and that I will actually be skinnier at the end, is just thrilling.