The thing with most of these ponzi schemes is that it is the late investors who end up losing all their money. If you are an early investor, who is intelligent enough to decide what is the best time to pull the plug, then these ponzis and HYIPs can turn out to be a boon for you. Off course, the risks are there, so be cautious.
Unfortunately, most people consider themselves to be of "above average" intelligence.
I have seen a lot of people knowingly investing in ponzis and then whining when it goes bust.