Bitcoin may or may not be a good investment, but it certainly is not a Ponzi Scheme
Suggest an addendum to Godwin's law.
If a discussion continues long enough of any new kind of intangible asset that appears to bring in speculators and exhibits S-curve like growth, there will eventually be mention of Ponzi Schemes.
Of course a key difference with a Ponzi Scheme; is that, in a Ponzi Scheme, the person running the scheme gets paid immediately, after soliciting investors, AND there is a formal promise of an outrageously good return on the original investment, used to promote/solicit investors, There is often deception in the form of statements with false returns, and existing capital, or funds provided by soliciting more investors are used to meet the payouts of return to earlier investors..
Since Bitcoin doesn't work like that... BTC itself is not a Ponzi scheme.
Of course.... you might claim one or more of the BTC exchanges are "Ponzi schemes"; if they are promoting Bitcoin, and suggesting that people "convert their dollars to Bitcoin" at a certain rate, with a promise of a return ---- then the people who convert Bitcoins back to dollars are paid, using the funds provided from earlier investors, while the exchange takes out a fee in dollars and a fee in Bitcoins every time an exchange happens.
However, in this respect..... all FOREX brokers could be thought of as Ponzi schemes, if they promote any currency, to inhabitants of a country that use a different currency.