It was originally a friendship/trading agreement in its early days, formed after the 2nd world war.
It's now become a beaurocratical nightmare, with designs (in mine and others opinions) of becoming some kind of federal state.
There are a lot of people across all European countries who are now wishing it hadn't gone so far. It's a completely different situation than the USA.
I'm glad we kept our Sterling, however bad things might be just at the moment. I'm sure that there are some who wished they hadn't given up their currencies.
Thor, would you ever agree to having your Dollar replaced to some new currency, with no local control over your monetery policies?
I suggest you Google it for more info. It's quite complicated and causes a lot of heated debate on both sides.