However the value of bitcoin is mainly driven by speculation at this point, a currency that's so volatile is very difficult to use if you're a business. At best you accept it at the current exchange rate and you immediately convert it to fiat. Anything else is very risky.
But that might be temporary, eventually the exchange rate might stabilize, at which point speculation could be vastly reduced and it would work better as a currency. That's kind of a chicken-and-egg problem though, in order for the exchange rate to stabilize you must attach a real world value to the currency by buying and selling things, paying taxes etc... But that won't happen as long as the exchange rate is so volatile.
I'm also very much unconvinced that a deflationary currency can work at all in the long run or that bitcoin will end up fixing the problems its proponents say it will, but that's up for debate.