Of the people I know It's roughly an equal split between speculation on its value and longer term interest in its potential as a low friction, decentralised currency.

Not to say it's not some peoples motivation but I think that they're probably in the minority. There was a piece which was on the front page of HN a while back (can't find the link right now) which estimated the total revenue going through silk road and it was a fairly small fraction of the total bitcoin transaction volume if I remember rightly.

FTA: "From a public relations standpoint, this is a positive sign for Bitcoin: the legal Bitcoin economy is now almost certainly larger than the illegal one, especially when one takes other merchant services like WalletBit/BIPS and Coinbase into consideration, and is growing at a much faster rate."