According to this TechCrunch article, 87% of surveyed economists are bearish on Bitcoin. So we can speculate about cost of adoption, velocity of money, deflation, and volatility all we want, but it seems that the people with PhDs who really know this stuff aren't optimistic.

Still probably a good idea to invest in Bitcoins some, just keep a diverse portfolio.

Edit: these economists don't seem that bearish, and they also seem to think they are pretty clueless.