Stuff is valuable because it is either useful (like hammers) or we think it is pretty (like Picasso paintings); the more rare, useful, and/or desirable something is, the more valuable it tends to be.
We know how rare bitcoins are. It remains to be seen how useful they are. I think they're pretty useful as both a convenient way of paying for things and as a long-term store of value, but it will take time to convince most people that they're useful for those things.