In an economic sense,
An INVESTMENT is the purchase of goods that are not consumed today but are used in the future to create wealth.
In finance
An INVESTMENT is a monetary asset purchased with the idea that the asset will provide income in the future or will later be sold at a higher price for a profit.
I really don't want to turn these into an economics class but the both definition of what investments are clearly should settle the argument of whether BTC is truly an investment or not.
Actually it is really a investment, since the value of bitcoin is really high and volatile. Just so happened that it can be trade to any goods or products online so some came up to the term currency, aside bitcoin can be converted into fiat so others think that bitcoin is also a kind of currency. Bitcoin slightly falls on currency category because of value but the truth is the bitcoin as a whole is a investment, a speculative investment,