My opinion on these matters is rather complex, because we are dealing with rather complex matters (now here's a wonderfully palindromic sentence!).
As a historian, I need to point out what I think to be the origin of Germany's economic misery. You see, Germany has been in recession for the last twenty years, ever since reunification. That is an undisputable fact. But that is not the fault of the Euro, and I'm going to lean out a bit further and say that it is not entirely the fault of reunification either.
The origins of Germany's economic crisis have their roots way further in the past. They begin with the mid-1960s, when the upward trend first threatened to stall: The country was reconstructed (note: I am talking solely about West Germany here). Reconstruction was what gave the country its economic boost in the first place, and it was done the way Germans were reputed to do it, efficiently and precise. To compare, the Italians did it more chaotically, which in the long run proved to be more profitable, because they kept their economic boost running much longer that way.
The Germans managed to keep the growth going nevertheless, because they had the significant advantage that "Made in Germany" is something of a quality label. German products have always been reputed to be efficient, robust and of high technical quality; this is the reason why Germany has been the number one exporting country worldwide consistently until only a few years back when China topped it. So, by the 1970s, Germany's economic income relied purely on the profits made from export. This is what caused the economic instability, because the oil crisis of 1973 and 1979 became a significant drawback for the Germans, because they could sell less products on the international market, which drove a million people into unemployment by the end of the 1970s. Despite this however, German profits were still so exorbitantly high the country could afford to pay for the wellfare and maintain its economic growth. Moreover, the number of unemployed stagnated and the Kohl administration employed its own brand of Thatcherian economics that seemed to promise to keep Germany as rich as it was, despite increasing corruption and little economic foresight.
Kohl's lack of foresight came back at him however, when the Berlin Wall fell. You need to consider that there was practically no forewarning; the waves of East German refugees did not appear until the summer of 1989, the mass movements inside East Germany did not start until October, and the Wall fell in November. It was not until that summer that everybody realised that East Germany was bankrupt, which was what made the masses flee in the first place. Kohl, like all other chancellors before him, maintained the long-termed goal that Germany was to be reunified. But nobody had expected that reunification would take place with zero forewarning within a single year (the Wall fell on 9. November 1989, reunification took place on 3. October 1990), and much less that Germany would incorporate such a large territory that was completely run to the ground, economically. By 1992, there was simply nothing in the east that worked efficiently anymore, virtually no factory producing any sort of output. The East German mark was traded in for the West German one at a course of 1:1, while the actual value was not even close to that. The amount of money burned in the process of reunification is stupefying, and the rise of unemployment was horrible; many former East Germans migrated to former West Germany, depopulating entire regions and boosting unemployment in the west. All this could not be paid for by an economy that made profits only from export, and while German produce and exports have actually been rising since reunification -same as profits-, the country fell into a recession it has not recovered from since. Unfortunately, the depopulation of the east has made attempts to reconstruct it fail, mostly, hence creating no new sources for revenue as it had been in West Germany after the war.
The Euro may have actually had the potential to counter these developments and help Germany get back to its feet, if it had been handled properly. The truth is that it was rushed - conceived and introduced within a mere decade - and therefore to this day carries some fatal flaws. The first one is that too much attention had been paid to individual national sentiments. Better said, too much effort was made to emphasise that countries like Portugal, Spain and Greece are part of the European Union. The Euro should have been introduced in stable economies like the BeNeLux, Ireland (at that time), Sweden, Finland, Denmark and major countries like France, Italy and Germany first and then the Euro Zone should have gradually been expanded. While France, Italy and Germany all are suffering from recession, the new currency would have eased investments from other Euro partners without bringing them down, because their economies are still immensely well developed. Other countries should have caught up with that development before being allowed to join.
Then, there is another, much more inexcusable flaw with the Euro. If you are going to have a common currency, you also need a common policy going with it - in other words, one single European department of finances. As it is, all members of the Euro Zone share one currency, but they individually free to do with it what they like. It is like a big family with a single bank account, and everybody is free to draw money from it whenever you like. Nobody does that, you have one single master plan, determining who gets what and when. Let's say you have mom, dad and five kids. Assume the parents earn the money and give each of their kids unlimited access to it at any time, with no efficient control: A month's income would be gone within a day. Nobody in their right mind would manage their family like this. Now imagine this family consists of 17 countries with 327 million people, and there are no lessons to be learned. Some countries know how to deal with this matter responsibly, others don't, and we see the consequences right now. Greece blew a shitload of money and everybody else has to pay. Ireland did the same, Portugal is coming up, and so are Spain and Heaven forbid, Italy.
I am a staunch advocate of European unification, more than anybody else I know, but the way things are handled right now, I would rather blow everything to pieces and start again.