It's a giant ponzi scheme:
In a Ponzi Scheme, the founders persuade investors that they'll profit. Bitcoin does not make such a guarantee. There is no central entity, just individuals building an economy.
A Ponzi scheme is a zero sum game. In a ponzi scheme, early adopters can only profit at the expense of late adopters, and the late adopters always lose. Bitcoin can have a win-win outcome. Earlier adopters profit from the rise in value as Bitcoin becomes better understood and in turn demanded by the public at large. All adopters benefit from the usefulness of a reliable and widely-accepted decentralized peer-to-peer currency.
It is also important to note that Satoshi Nakamoto, creator of bitcoin, has never spent a bitcoin (other than giving them away when they were worthless) which we can verify by checking the blockchain.