It looks like a Ponzi scheme because, when every potential bitcoin users of the population will have exhausted all of their "potential of speculation", there will be nothing left to sustain the price of the bitcoin.
It's constantly happening and what we see next is that the price recovers itself, so what would this be? Maybe a sustainable Ponzi scheme that is always recreating itself even stronger? Increasements and decreasements in price are part of Bitcoin's nature, and although there is speculation, there is utility for Bitcoin, so it doesn't seem a Ponzi.
Now that BTC price is hiting awesome numbers I have seen experts trying to blame it, they say Bitcoin is pure speculation and for this reason it can't be considered an investment.