It doesn't matter it it has value or not, the bottom line is if you agree with each other that THIS thing represents your efforts and goods for trade then it's currency. People traded with sticks with notches cut in them for a time because the king created them and ordered that only they could be used to pay taxes, and they worked great for trade because of the way he made them so they couldn't be counterfeited.

The only things you need for good currency is first to make it so it can't be counterfeited. Bitcoin's definitely got that covered. The next thing you need is to scale the amount in circulation according to the needs of trade. Since bitcoin has a sort of soft cap of how much can be generated (bitcoin becomes exponentially more difficult to generate), I think this is also covered.

The danger is if there is too little of it then a few can eventually amass it and then control it. That's what I'm not sure of with bitcoin. This is what happened to all those computer video cards, a few created massive bitcoin farms with them to amass it.