I believe that criminals use the coins not as an investment but as a payment system that can’t be tracked. Normal people tend not to use it as payment anymore, because the Bitcoin value goes mostly up, and it is a hassle to constantly have to check the going rate.
There is no doubt in my mind that Bitcoin is a high risk investment, and that every investor could lose a ton of money with it, but there is no Ponzi scheme here: there is no borrowing involved, or attempt to lure more people to invest, and nobody is behind all this raking in the profits, except the investors themselves, and the miners.