Careful of charges with Cirrus/Maestro! Check them out first - they can be high.
I'd say:
- use credit card for purchases as much as possible, as that tends to give the best rate
- they use credit cards much more in the US, even for small purchases
- look into loading cash into the credit card before you go and then using that at cash machines (though you may have a liability if the cars is stolen whilst it has a positive balance)
- If asked when getting stuff on the credit card if you want to pay in Euros, the answer should always be "no". Pay in dollars and let Visa or whoever convert
- Draw cash using from ATMs laser or credit card (loaded with cash to avoid interst payments) whilst there, but make sure you know what charges are
- apart from that, changing cash (the folding stuff), regular banks are usually the best place, and easier to do before you go