As for previous commenters, the fact that the value of a Bitcoin might be worth ten times more (or less) what it is now in a few weeks has more to do with its volatility than its depreciative nature.
Why would you ever risk money by lending it when you could just "hide it under your mattress" and make money? It's appealing to those who have the option to do that, but I believe it's highly suboptimal from an economics point of view.
The funny thing is that even though it's bad economically, its deflationary nature is good for the uptake of the currency, because, provided more people start using it, those who are already using it become richer (which is why some accuse it of being a pyramid scheme).
On the other hand, its mid-term value depends on why people are buying into it now. At the moment, I see four groups of people: 1) the idealists who believe it's simply a more perfect form of currency that can't be corrupted by governments et al; 2) the technologists who think it's a neat idea; 3) the speculators who think it's going to go up because it's going up; 4) the users who find it more useful than regular currency (like those on Silk Road). I can't help but think that the speculators are the biggest group at the moment. But I might be wrong.