I don't see it as a currency, because it's not behaving like a currency, or designed for the purposes that currencies are designed for.

It sort of mimics as a currency, as you can use it even easier then fiat money (from buyers point of view), if you are using bitcoin payment services. But currency has more purposes then using it for trading goods and services. It can be used to stabilize whole regions in times of crisis. Because if people would still use gold based currencies, there would not be a buffer, and if there's no buffer, there could be panic and chaos. And taxes couldn't fix that, so it would lead to government seizing companies and commodities by force.

Bitcoin is outside from that. It can be used in similar ways, as it's fast and quite liquid these days. But number of services actually accepting it as a payment (not accepting via third party payment system), is low.

It's a form of investment, but promise of growth of it is still very much relying on loose regulations of it.