There are two things that are required for a currency. The first is that you can make transactions in it, it's a medium of exchange and the second thing is that it's a storage of wealth. Those would be the two purposes of a currency.
Bitcoin today, you really can't make much transactions in it and in terms of a storage of wealth it's not an effective storage of wealth because it's it has a volatility to it unlike gold. Bitcoin is a highly speculative market.