The euro is a quite different issue. Back when the single currency was being contemplated, the fundamental concern of many economists on this side of the Atlantic was, how will Europe adjust to asymmetric shocks? Suppose that some members of the euro zone are hit much harder by a downturn than others, so that they have much higher-than-average unemployment; how will they adjust?
In the United States, such shocks are cushioned by the existence of a federal government: the Social Security and Medicare checks keep being sent to Florida, even after the bubble bursts. And we adjust to a large degree with labor mobility: workers move in large numbers from depressed states to those that are doing better.
Europe lacks both the centralized fiscal system and the high labor mobility. (Yes, some workers move, but not nearly on the US scale).
To be sure, America has at least minor-league versions of the same problems: we are having fiscal crises in the states, and the housing slump has depressed mobility in the recession. But we’re still better able to cope with asymmetric shocks than the eurozone.
Was the euro a mistake? There were benefits — but the costs are proving much higher than the optimists claimed. On balance, I still consider it the wrong move, but in a way that’s irrelevant: it happened, it’s not reversible, so Europe now has to find a way to make it work.