An author on Slate wrote on this. I'm not sure what to think about it, but I'll leave you to interpret what you will:

Felix Salmon and many others have pointed out that a currency cannot succeed with a supply that is fixed, or if it grows too slowly. A currency is used to enter transactions; the more transactions there are, the more of the money you need. As the economy grows, a fixed-supply currency becomes worth more in terms of goods and services, and people begin to hoard it—expecting that if they wait a little longer, they will be able to buy more. Once hoarding takes over, circulation ends, and with it the function of the currency. Hoarding accounts for the large increase in the value of bitcoins[...].

An even more fundamental problem with bitcoins, and indeed any private currency, is that there is no way to limit its supply. True, bitcoins cannot be manufactured beyond the limits set by Nakamoto. But there is no way to prevent future Nakamotos from creating bitcoin substitutes—say, bytecoin, or botcoin. If merchants are willing to accept bitcoins, they will be willing to accept the substitutes, especially as bitcoins become scarce and consumers scramble for substitutes. Nakamoto must have realized this because there are not enough bitcoins to substitute for the currencies around the world. The currency can only succeed if it is expanded or supplemented. But if there are no constraints on substitute digital currencies—and there aren’t—then the value of bitcoins will plummet as the subs begin to circulate. And once it becomes clear that there is no limit, people will realize that their holdings could become worthless at any moment, and demand for bitcoins and the other currencies will collapse, ending the experiment.

You can read the entire thing here:
http://www.slate.com/articles/news_and_politics/view_from_chicago/2013/04/bitcoin_is_a_ponzi_scheme_the_internet_currency_will_collapse.html