As a fiat currency, it is almost by definition a bubble. It's fundamental value is essentially nil, yet it's "price" is positive. In the current equilibrium, people seem to agree that Bitcoin is a good store of value (because it is perhaps limited in supply and ownership is verifiable). Fortunately/unfortunately, other cryptocurrencies are just as useful in this regard. As such, Bitcoin's value is something of a sunspot equilibrium. Any sufficiently big shock or event could shift the focus to another cryptocurrency or other product entirely.