"But who the h**l is backing the value of the Bitcoin? Sure, it has gone up like a rocket - but so did the price of tulip bulbs in 17C Holland"
Geez, you'd think people would be able to think for themselves rather than simply regurgitating whatever Paul Krugman told them.
Bitcoins are valued by people for many reasons: as an interesting phenomenon, for the ability to transfer value anywhere in the world almost instantaneously, for the ability to have total personal control of wealth, as an investment for the future, etc. The list goes on and on. Bitcoin is valued because of the unique things it allows you to do. Tulips, for example, don't allow you to transfer wealth to someone on the opposite side of the world outside of any govt controls. Trying to draw some analogy between the two is downright silly.