I can imagine bitcoins as a currency, but it is really hard for me to imagine it is as a commodity. Commodities need some sort of underlying value (currency doesn't) and bitcoin doesn't.
I disagree. The price of gold doesn't reflect a practical value. People employ it as a commodity as it has a limited supply, and do so based on the sentiment in other markets, not on a perceived rise in the underlying value of gold. I see gold's underlying value being its limited supply and as it cannot be duplicated. BTC shares the same characteristics, but is easier to exchange.Commodities need some sort of underlying value (currency doesn't) and bitcoin doesn't.
The recent bubble in gold is most likely due to speculation, but gold has an underlying value. As of now the majority of gold demand does not come from investment demands but from jewelery and technology. Here is a chart with the demand in tons
This is also a very new trend for gold. If you look at this PDF you'll see that investment demand for gold has gone from like 10-20% to 40% in the last decade.