If all humans used bitcoins now (Jan 2013) for most of their transactions, then a dollar would be worth about a billionth of a bitcoin (1/10 Satoshi).
If bitcoins take ten years to catch on that fully, and population rises only a little while worldwide productivity doubles, then you could buy 5 candy bars for a Satoshi (1/10^8 BTC).
A house today, starter home might be worth 10 000 bitcoins or so. Paying a 30 year loan payment in Bitcoins would require a payment of a billion dollars a month at some point.
So I don't see how you could have viable loans in bitcoins until the system has settled down at least 30 years from now.
And if you have 1 bitcoin today, it could become worth 100 million dollars purchasing power in 10 to 20 years. One inexpensive coin worth an hour's labor.
So the system has an enormous potential reward for hoarding--far beyond working for a living, or any remotely normal successful investment.
How is hoarding to be prevented?
And I think this system is even more unsound than fiat money, albeit in a very different direction. But it IS likely to lead to drastically improved money in the long run.