Money is the general medium of exchange. It is the most widely-traded good in the economy. Because of this, economic calculation, i.e., accounting, is done in money. Bitcoins, then, are not money.
Money substitutes are at par, on demand redemption claims for money. Banks that issue checking accounts will redeem them at par (i.e., at the fixed, face value of the claim), on demand for cash. That’s why merchants throughout the economy accept them in lieu of money itself. Bitcoins, then, are not money substitutes.
In a market economy, people are free to try new mediums of exchange, If BTC is such an attempt, it is not the first and, presumably, will not be the last. Here are few articles giving economic analyses of bitcoins: