Is Bitcoin a ponzi ? :
Bitcoin works just like any other stock. Supply and demand. Just like with apple shares , when people buy it the price increase , price goes up , when people sell it , price goes down. So no , Bitcoin is not a ponzi.
Is bitcoin a bubble ? :
People often compare bitcoin growth with the dot com bubble in the early 2000's. But if you take a closer look on the NASDAQ historical chart , you can see that yes it crashed in 2000 but now we are in 2018 and not only the market recovered but it is now much higher than it was at the peak of the "bubble". In addition , companies like Amazon who suffered big drops of their shares during the dot com "bubble" , are now worth much more than they were worth at the peak of the "bubble".
Also keep in mind that when bitcoin reached $10 although it was was worth only $0.05 at the begining , some people screamed bubble. When it reached $100 , even more. Look at the greater picture.
Bitcoin is a store of value and arguably a decentralized international currency. And cryptocurrencies have the potential to replace the "shares" system , often complicated for most people. Soon every company might want to issue its own token. Are Bitcoin and cryptocurrencies in a bubble ? No. Cryptocurrency is just another market , having Ups and Downs , very volatile , but over the long term , it's bullish.