Now bitcoin has built-in a hard limit. Which makes the currency, inherently inflationary. I have no evidence, but I believe that this was a strategic choice by Satoshi. Why would you buy a currency made out of cpu cycles, with no backing if not for it's future value? Without being a state (e.g. selling trust) or having huge amounts of gold to back up a new currency, the only reason someone would wanna buy it, is indeed, the future value that could hold. Hence, Bitcoin or any cryptocurrency can work as an asset but not as a currency given the current design.
Y. Varoufakis, spoke of a centralised digital currency in his solution for the Greek crisis. That could be a digital currency, that anyone can use. But was a fully controlled digital currency issued in the form of IOUs by the state.
NOTE: Talking about adoption of a decentralised currency a-la bitcoin, by a state or group of states to replace entities like the FED or the ECB shows a poor understanding of the most basic principles of the financial systems.
Its completely irrelevant what the price of Bitcoin is for its usefulness as a medium of exchange.
This medium of exchange aspect is very important because it makes banks no longer special since its possible in time to make payments without a banking system. Thus the role of the state in regulating banks can disappear eventually - no deposit insurance, no bailouts and no too big to fail.
Credit can be provided by funds because the information asymmetry of banks no longer applies when you can port your banking records via API.
Bitcoin works reasonably well as a medium of exchange, as long as transactions are confirmed relatively quickly.
Bitcoin prices are pretty much always tied to the price in US dollars or some other fiat currency. Online prices can be changed dynamically, so Bitcoin doesn't really need to be its own unit of account.
But it never really works as a store of value, due to its volatility. I have about $20 in Bitcoin today, but I have no idea what that will be worth in a week, so it's risky to keep any large amount of money in Bitcoin. The safest way to buy things in Bitcoin would be to buy it, and then immediately send it to whomever you're buying from.
So I think that Bitcoin will be much more usable when its price stabilizes, and there's much less friction with each purchase.