Successful currencies meet three criteria:
(1) they are in widespread use as medium of exchange
(2) they serve as unit of account
(3) they are a stable store of value.
Most sovereign currencies fully meet all three criteria.
What about Bitcoin?
It's very well but it's not safe enough to make people trus bitcoin.
If the security is more improved, i'm sure bitcoin will be a global currency in the future
The function of Bitcoin as currency is it can used as payment in other site that accept bitcoin, even its not yet accept of many country but still there are some country accept it as payment. But i think there are still many people who invest to it than used it as payment because they know they can possible get huge profit. And I think Bitcoin will be more popular and become well-known so that some store are accept it as payment.