Under the traditional paradigms of economic analysis, Bitcoin is most certainly a bubble. However, I have been tempted to give it the benefit of the doubt since there is really no historical precedent for the economic behavior of cryptocurrency. Now consider the definition of an economic bubble: "An economic bubble is trade in an asset at a price or price range that strongly exceeds the asset's intrinsic value." This raises what is to me the core unanswered and perhaps ultimately unanswerable question about cryptocurrency trading: What is the intrinsic value of a Bitcoin?
My answer is: Once upon a time, the intrinsic value of a Bitcoin was commensurate with the computing power and electrical power used to mine it. But now that there are alternative cryptocurrencies which improve substantially on Bitcoin in one or more major facets: essentially nothing. Any apparent intrinsic value that Bitcoin has is actually retained momentum from being the first cryptocurrency.
Bitcoin serves one practical purpose, which is allowing individuals worldwide to transfer money without the encumbrances of currency exchanges, transaction fees, and administrative overhead. Some hardcore advocates will extol the glories of Bitcoin as the most secure and anonymous means of payment available. It may once have been that, but these days that cannot be considered likely because Bitcoin has become mainstream, with all of the regulation and monitoring that entails.
However, Bitcoin is currently traded primarily as a means of speculation, which has enormously inflated its price. Eventually, the market will correct itself by popping this bubble. Then currencies more suited to actually being used a means of payment, such as Ripple, will take hold. Ripple is the only major cryptocurrency I can recommend speculating on, but even then, the hopeful investor should regard Ripple not as investment but gambling, and therefore never play with more than they can lose.