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?
As of this time Bitcoin is more popular as an investment asset and not as a currency wherein it is really what is intended to be by the legendary founder Satoshi Nakamoto. Though it is not known by late comers as a currency but still the community knows it's purpose of existence and I myselfuse it as a currency to buy prepaid loads for my data connection and I find it convenient because it is super fast unfortunately Bitcoin is unstable due to volatility.