The euro is not an economic project, but a political one, so it's not enough to just look at the economic benefits and downsides of it. It was never meant as a 'get rich, guaranteed!' scheme for everyone.
People sometimes forget the core reason why the European union was founded. As a reminder, look at the Schuman Declaration from 1950:
"The solidarity in production thus established will make it plain that any war between France and Germany becomes not merely unthinkable, but materially impossible"
This proclamation came at a time when in a period of one human life time, three generations of young men had had the opportunity to get themselves killed in a major war between France and Germany. Looking a bit further back, one can see that this cycle has actually been going on at least since the 17th century, or even longer. The ideal of the European union was not boosting the economy, but to promote and solidify peace. Until very recently I was convinced Schuman and Adenauer had been spectacularly successful in their goal. Eighty years of peace in Europe has been the longest period I have been able to find in modern history, and another war between France and Germany seemed unthinkable to me.
All the other things the EU does, stem from this ideal. The common market serves the ideal of peace. EU regulations coming from Brussels serve to make this common market work and prevent a race to the bottom, where every state is forced to lower their taxes and ease their regulations in order to prevent losing business and industry to other countries. The Euro serves this same purpose, namely to prevent a race to the bottom in a kind of devaluation war or currency speculation.
Now whether the Euro has been successful at this goal, I'm not qualified to judge. It's also difficult to replay the crisis of 2008 and see how it would have played out without the Euro. Whether it would have been worse or better, longer or shorter, who knows, but to call it a failure just because it didn't make us as rich as the Americans is too simple.