In the case of Europe, I think a universal currency is best for a continent such as Europe since it promotes more of an internationalist direction than each European country has their own currency. History in Europe has shown where ultra-nationalism and fight for hegemony lead to (World War 1 and 2) so I think for the sake of humanity, a universal currency is best.
I definitely think that more countries in Europe should use the Euro. I think that it would make business between countries in that continent better as well as help the economy to grow. I don't think that countries outside of Europe to use it though since there is no benefit to it.
No, more countries should not use the Euro, because if so many nations are on that one currency, the nations will have very large swings when one nation has a problem. England especially should maintain its independence, rather than being subject to the economic swings that the other nations have.
On its face, the euro is a good idea, but like most good ideas, the longer it is in effect, the more problems crop up. If only a few countries using the euro are stable, as is the case, it becomes inevitable that the idea will wither away. Having a few countries prop up all the others is a recipe for disaster.
More countries should not use the Euro. The value of the Euro is decreasing everyday and it would be a mistake to make new countries use this currency. I think they should continue to use the currency that they have because the Euro is eventually going to bottom out and fail.
When the Euro began it seemed like a relatively good idea. But the cultural barriers between countries when traveling across Europe seem to have been eroding since then. The pleasure of driving across Europe is experiencing different cuisines, languages, landscapes, emotions, cultural proclivities, and currency has everything to do with these factors.