Europe has had more than its share of botched and of successful currency unions. The Snake, the EMS, the ERM, on the one hand - and the British Pound, the Deutschmark, and the ECU, on the other. The currency unions which made it have all survived because they relied on a single monetary authority for managing the currency.
Counter-intuitively, single currencies are often associated with complex political entities which occupy vast swathes of land and incorporate previously distinct -and often politically, socially, and economically disparate - units. The USA is a monetary union, as was the late USSR.