>Why is Bitcoin a good currency again?
It isn't, but almost anything can work if the community of users is small enough. As for why it isn't, we need to step back a bit.
In a barter economy an exchange occurs when two individuals each produce something which takes time/labor/capital and which the other person values more than what they're giving up. For example, if I grow an apple, and you grow an orange, and we trade, it is clear[1] that I value the orange more than the apple, and you value the apple more than the orange (otherwise we wouldn't have made the exchange). The value of those items is individually subjective[2], and the supply of those items is constrained by market forces (e.g., alternate uses of time, labor, and capital).
A money economy emerges from a barter economy when individuals accept in exchange something they do not want for itself, but with the anticipation that they can use it for some future exchange. It is critical that the commodity in question be desirable eventually. If no one ever wants it, then it wouldn't emerge as money. (Important Aside: once a commodity begins to function as money, part of the demand for the commodity will be as money, thus there is positive feedback loop, but some non-money demand needs to exist to get the process running. This is why all functioning fiat currencies have their roots in a commodity money.)
It's important to notice that commodity money has some inherent constraint on its creation (i.e., its supply curve is bounded by market forces), and that it satisfies the wants of others (i.e., there exists some demand curve). For example, the gold supply is constrained by the cost of mining/processing new gold, and the demand affected by its consumption into produced goods and demand to hold "cash balances" of gold.
So far as I can tell, bitcoin is a scheme whose total supply is constrained only by an exogenous, artificial limit of 21M coins, and not by the demand for alternative uses of its factors of production[3]. There can be, at most, a marginal value[4] equal to the marginal cost of producing a coin, which appears to be nothing but otherwise-idle computer time and some electricity.
But far more importantly, this is a scheme whereby artificial scarcity is created (e.g., it takes n hours to produce a coin) but no thing of value is produced. This would be akin to inventing a system of money involving sticks whittled into perfect cylinders. Sure it might take a long time to create one, and it might consume resources, but if the end result doesn't have any value to anyone, it isn't money, it's just waste.
"The curious task of economics is to demonstrate to men how little they really know about what they imagine they can design."
[1] http://en.wikipedia.org/wiki/Revealed_preference
[2] http://en.wikipedia.org/wiki/Subjective_theory_of_value
[3] http://en.wikipedia.org/wiki/Factors_of_production
[4] http://en.wikipedia.org/wiki/Marginalism