Apologies if anybody has covered any of this already, but I didn't bother to read any of the posts after the original.
In order for any non-fiat entity to be truly accepted as money (a general use medium of exchange), it must have originally established a market value in barter. Remembrance of the recent price (market exchange value) of money is absolutely necessary in order for people to assess and trust its current value, and this can logically be traced back to its initial value as a commodity in barter. Once it is accepted as the preferred medium of exchange, its value as money will remain even if it loses its use as a non-monetary commodity. Whether or not it needed to have originally had a non-monetary use is open to speculation.
The previous reasoning summarizes the "Menger-Mises Regression Theorem." Both Menger and Mises often wrote that money needed to first have a value as a non-monetary commodity; however, the "flying currency" of the Tang Dynasty provides a historical counter-example as it came into being for specific use as a non-fiat medium of exchange and not as a non-monetary commodity. The true crux of the issue, though, is that media of exchange require exchange values emerging through market activity.
Qualities of a commodity which allow it to emerge as a desirable medium of indirect exchange include:
It is easily recognizable
It is scarce
It is durable
It is easily divisible into smaller units
It is storable at low cost
It is fungible
It is easily transported
Its supply is relatively stable relative to attainment or production of other consumer goods
So what about Bitcoin? It certainly meets (in a way) the criteria above as a desirable medium of exchange. But does it satisfy the regression criteria? In a sense, yes. It did have a trial period within an ever-growing circle of users, and the speed of transaction possible in the digital age allowed as many transactions to be made as would have taken decades during the days of barter when gold was established as money. The benefits of Bitcoin transactions which were tested and subsequently accepted include anonymity, low regulation of transactions, fixed growth rate and limited supply of the currency. So "security in exchange" seems to be the "commodity" which was tested and which eventually enabled the underlying currency to establish a dollar value on the open market, where its demand grew very rapidly. Ergo, some people consider it a non-fiat money. Others claim the "currency" itself is just a toy since it never had a non-monetary use (but remember the Tang Dynasty cash was born in the same way).
Will BTC retain value or is it a hype-driven bubble? Could government eventually control and vitiate the currency as governments have always done (historically through debasement or creation of an easily-inflatable fiat legal tender)?
It does have its problems relative to other methods of transaction; for example, MasterCard, though it transfers fiat currency, can handle hundreds of times as many transactions in the same time frame, which makes it more desirable for use in a global market, at least among the masses who are blissfully unaware that it is transferring inflatable "money." And new cryptocurrencies are emerging which may prove more desirable. Whether or not Bitcoin tanks, cryptocurrencies will likely be here for quite awhile.
However, there always looms the specter of central banks. If central banks can approve their own versions of digital currency, it's game over. Governments will be free to extend their financial domination. Here we can see the added value of having a medium of exchange which has been a commodity in the non-monetary sense and which cannot be inflated at will. And you thought the first few paragraphs up there were unnecessary! Not at all. Have a listen to this (length 5 hours, 9 minutes, but worth it):
www.youtube.com/watch?v=RfjoDgLDfaU&t=2658s