Not only has it been a massive boost to the German economy, it has been at the explicit expense of the economies of Greece, Italy, Spain, etc. And how people can see that and be ok with it, I don't understand.

The EU under the Euro is the Fourth Reich. Nothing more, nothing less.

They could devalue their currency if they wanted to, if they had their own currency instead of the Euro

Which is what Greece, Italy etc would be doing if they weren't in the Euro. Germany wouldn't devalue as that involves increasing money supply which would drive inflation, the German post war banking system is built to have a massive fear of inflation.