The problem everyone misses is culture. America prospered as it was a large geographically well placed and well resourced state which most importantly had a common language under-lying its open trade and movement. This is why the main economic risers are not groups of countries but specific large countries.
The currency union never met the requirements because it did not have a common language but also because the social fabrics of the countries are so different, they would never be able to agree regional distribution or common taxation. In fact the very idea is not what many Europeans want from Europe.
The modern UK is a great example of this. The very ideas of the currency union and redistribution are under threat by different social views. Europe does need to rethink the Euro. It was sold by those who yes wanted the 'currency union' to those who wanted integration and economic prosperity, what was not reflected on was that the European Union is a political 'project' it is not the norm of Europe.
However. And its a big fucking however. Most of Europe is fine. Surely theres ups and downs. Ireland was the big fuck up, a well of country smacked more heavily by the crisis thrown into huge debts and would have been better with the Sterling monetary policy; Spain and Italy are muddling through and youth unemployment is pretty poor. Honestly? For the Northern and Central European nations the Euro has been a positive, maybe not a big one economically but socially and diplomatically it has driven integration.
Greece has never been the concern. Its the concern of their affect on how people in other countries perceive their debt and how foreigners perceive their debt.
There's an argument that an optimal currency zone for Germany, Belgium, France, and some of the other northern European countries would've been fine. No one else though. It's ludicrous what's happening now.