The success of a currency is not judged by its relative strength in the markets. That would mean it changes all the time. All currencies rise and fall to reflect the markets' judgement on economies.(Original post by Quady)
The euro has been pretty successful from what I can see, much stronger than stirling than when it was launched.
A currency's strength lies in its ability to mitigate risk to its country's economy, and to make adjustments that benefit as much of the economy as possible. There is no doubt that the euro can not balance the benefits across all the eurozone, and this is demonstrated by the north v south problems, caused by economies that are not in sync (and in Greece's case, not on Planet Earth). The currency is being asked to do too much across too disparate an area.
The eurozone is still just one step away from another crisis.
Sterling's strength is that it covers a very homogeneous area, with a much better balanced area of coverage, as well as far more unified control.
Perhaps the independent Scots could adopt Stirling as a currency. The major unit, the wallace, would be composed of one hundred bruces, and the currency could form a bridge between the pound and the euro.
Start yours and you could win £150!