How has the Euro been beneficial?

With the current financial crisis, we’re often told that, had we still been using the Guilder in the Netherlands, we’d have been much worse off, perhaps even Icelandic situations. And indeed, the value of e.g. the British Pound has plummeted. However, why exactly is this a bad thing, and what does the Euro have that makes it more stable than, for example, the Guilder?