"It's an absolute bubble – there's no question in my mind that it's in a bubble. Sure you can make money in bubbles any time but you have to get out. Let's not forget that the total market value of these cryptocurrencies is $180 billion or so, maybe a little less now -that's tiny compared to gold," said John Hathaway of Tocqueville Asset Management.

"Bitcoin is the best example of speculative bubble. And I think that has to do with the motivating quality of the bitcoin story. And I've seen it in my students at Yale. You start talking about bitcoin and they're excited! And I think, what's so exciting?," noted Robert Shiller, the Yale economics professor and author of a seminal book on speculative manias, Irrational Exuberance.

"Bitcoin is a highly speculative market. Bitcoin is a bubble. It's a shame, it could be a currency. It could work conceptually, but the amount of speculation that is going on and the lack of transactions hurts it," stressed Ray Dalio, Bridgewater Associates founder.

"This is going to be the largest bubble of our lifetimes. Prices are going to get way ahead of where they should be. You can make a whole lot of money on the way up, and we plan on it," believes Mike Novogratz, ex-manager at Fortress Investment Group LLC.

"I think Bitcoin is a bubble. I just don't know when or how much it corrects. When everyone is bragging about how easy they are making then it is a bubble," said the billionaire Mark Cuban.

"From what we can identify, the only reason today to buy or sell bitcoin is to make money, which is the very definition of speculation and the very definition of a bubble. The history of finance shows that this kind of phenomenon rarely ends well," think Tidjane Thiam, Credit Suisse CEO.