Every person I have known to do this diet gains the weight back.
My mom: Lost 16 lbs in a month. Was lethargic the whole time, went to bed at 7 pm every night. Felt starving all the time. Gained the weight back the month after. Is now at her heaviest.
My mother in law: Has done HCG at least 10 times over the last few years. Claims she feels great and has no side effects. Loses about 16 pounds every time. Gains back all the weight immediately. I don't know why she keeps doing it. She looks good WHILE she is doing it, but she looks the same as she has for years and so it is not helping her to whittle down her weight or anything.
Sis in law: Tried it for 3 days. Almost passed out at work and immediately went off the diet.
My other SIL's mom and dad: Both lost a lot of weight. Both are back at original weight.
Good friend: Lost about 20 lbs. Is now back where she started.
I understand the desire to get fast results. In the end, though, it just seems like it would be a better idea to directly throw your money in the garbage.
By the way, the HGC diet is very extreme. 500 calories a day, many fruits and veggies are off limits. I believe you get a small portion of chicken or fish each day and then some fruits and veggies. What I would suggest instead is eliminating refined flours, sugars, and white foods like white rice and potatoes. My SIL has lost 16 lbs in the last 2 months doing just that. I lost 10 lbs in a month doing that. It is a lot more doable and costs a lot less :)