I know this is being pushed by kevin trudeau of the "establishment is hiding the truth from you but I'll tell you as long as you pay to join my website" fame, so I admit to being suspicious of the plan. in his book he does mention the originator of the diet (it's not trudeau) and such a diet concept has been around for quite a number of years but has always been considered controversial for the reasons that lorka mentioned.
Having said that, 500 calorie diets will work whether or not you have an injection of hcg as the diet is more or less all protein and a bit of very low glycemic fruit and vegetables. I imagine the hcg is to help you from being hungry and to speed up your metabolism. In Canada, and a couple of states on the east coast, we have a similar diet Dr Bernsteins (not the diabetes doc, this is another one) that has been around for a long time and does something similar. in this case the injections are every 2 days and are allegedly comprised of b12 and b6 or a b complex if you are getting a cold.
it works but the real problem is once you stop the b12/6 injections and start maintenance you have to be SUPER careful to add food in slowly, and by slowly I mean up it by 100 calories every 2 weeks or so otherwise it's rebound city.
I did Dr B and kept the weight off until this past summer when either hormones or the worsening of Gluten intolerance caused me to jump up 10lbs in 7 days

(and this was weeks after my vacation where I had had lots to eat and drink!

)
so I can tell you it does work but it's not an easy plan to follow. if you want to get an idea of what can happen to you on such a low calorie diet (but without injections) then google "kimkins" and read some of their stories. If nothing else the stories of the constant nausea aka "SNAP" is a turnoff but I will admit the part about the scam is both sad and interesting at the same time.