I think that they are an investment.

BTC cant stay at ~100$ forever if its going to be "successful".
With only 21 million coins, market liquidity won't be possible with only $2.1billion money stock.

The good thing is that Bitcoins ROI time is shorter than traditional investment methods.
Its still a gamble, and with all big rewards, comes big risks - just as it has been like at every step of Bitcoins existence.