The downside of the Euro is that it imposes a single bank lending rate across the whole of the Eurozone. This means that countries in differing parts of the economic cycle need differing interest rates, but they can't get them. So countries that need high interest rates suffer when Germany/France need low rates and vice versa.

In the US, the common currency and rates have been around for a long time. The country as a whole tends to be on an economic downturn or upturn, much more so than in the EU. It will even out in the EU, but at what cost to the individual countries?