I don't know if it's a Ponzi scheme but it's definitely a strange phenomenon.
Is there a bank where after you deposit your money, it grows in value if other people after you also deposit in the same bank? The more and more people deposit the higher your deposit becomes?
I know it's insane. The danger is that if more people start withdrawing your deposit will start shrinking.
This is one of the reason Bitcoin is not really money.
The other reason is that it's not a measure of value. Is there any one who will give their product or service price in bitcoins? The way the price of Bitcoin fluctuates, it's impossible to do so.