A lot of things have been said, and almost all of them have been answered to.
The fact different countries having different economical cycles is not an obstacle for a single currency. Or will you say that California economy is the same as Wisconsin one, for an example?
Here, in Spain, I am a Catalan. Catalonia is 10% of surface, 12% of population, and... 27%!!!! of the gross. We know quite well what does mean to help the weaker spanish regions: we pay 120% of the taxes above them (if they pay 100 we pay 120), and get in return aboput 80% of it (for every 100 invested there, only 80 are invested to us). And Spain is a small country that has less population than California alone.
The goal of a single currency is the armonization of all the different economic cycles, in order to create a bigger and more stable one, and there WILL STILL be some differences, but smaller. And the stronger areas will help the weaker ones sending money for financial balance. And, tell me something: is this not the same way the US Treasury balances the economy of the weaker states?.
It is the same here: Spain, Greece, Ireland, Portugal, have received lots of financial help from the EU. Help that, simply, came from Germany, France and Italy.
Some people here say that Germany had a strong Mark and now is not doing well because of the Euro.
WRONG: Germany is not doing well (sorry: They are doing VERY WELL, simply not as much as they used to...) because they have been economically funding and developping the former Democratic Republic. That's why.
Finally, something else:
Facial Value DOES NOT EQUAL Financial Value.
Were it so, the strongest currency in the world would be the Sterling Pound, followed by the Irish Pound.
But, although US$1 is worth only GBp66, the USD is quite stronger than the GBP. Even the German Mark (1DEM=31GBp, or, what's the same 1GB£=3.11DEM) is at least as strong (if not even stronger) as the GBP is. For getting 1 Euro, you have to pay almost 2,000 Italian Lire (1,936.27). On the other hand, you need only 200 Portuguese Escudos. (200.482) This does not mean that the Lire is so weak. Actually, it is (OK, it was, before the convergency) quite stronger than the Escudo.
Something is true: the Euro is not doing too well in front of the dollar lately. On Jan 1st, 1999, when the Euro was reconverted fromt he E.C.U., 1 EUR was worth 1.12 US$. Two weeks ago 1 EUR was just 83 USc. That's a huge difference. But I bet this is owed to the too strong Dollar right now, than to a weak Euro. Aussies, Brits, Canadians: has your currency lost some facial value in the latest two years in front of the greenback or not?
Do not forget that every 7 to 10 years, the USD gets into a finacial bubble that raises it against ALL the rest of the currencies of the world. Now, the dollar is in one of those bubbles. Maybe the strongest one after many many decades, it is true, but is going to get out of that. And for USA's sake..., because the ONLY advantage of the US$ being so strong is for the US tourists going abroad. For the whole rest, petroleum gets too expensive, exports from US decrease because they cost more, imports to the US increase because they cost less, and touristic income lowers because foreigners can't afford it.
What I'm gonna write is quite simple, but an artificial facial strenght usually leads, if kept for too long, to a financial weakness, due to deficit and trade balance with other countries.
It is time for the dollar to start descending procedures... I give less that 12 months from now to the parity EUR-US$, 1-1 or 1-0.95 either above either below.
Back to the topic, a convergency would be good for both economies. A cesion of a little bit of sovereignty is not a catastrophy.
But the solution is not USA adopting CN$, neither Canada adopting US$. The creation of some kind of "Ameros" would be balanced, kind, and fair. And would boost NAFTA even with Mexico joining the "Amero". Just count the gross product and the population of the three countries together...
They could even help small center american countries with much less effort than the Germans helping the whole Europe...