I once had the idea for my own "personal currency"... I would set the exchange rate... For some reason it didn't really catch on.

The article alludes to it, but in the UK, legal tender is a bit sketchy at best: http://en.wikipedia.org/wiki/Legal_tender#In_the_United_King...

The reality of this really comes home when you try to spend your Scottish Pounds in England (I got caught by this first time I went to Scotland).

Of course, the counter-example is the Euro - which the participating countries seem to grumble about a bit (it's blamed for inflating some prices)... but as a tourist it's a godsend.