Generally, with any credit or debit card you're getting a decent exchange rate. The rates are usually set by Visa or Mastercard/Eurocard.
I know Nationwide advertises this best rate thing... I've yet to figure what they do differently since their cards must work on some system (i.e. Visa/Visa Electron or MasterCard/Eurocard/Maestro!)
The only thing to check is if your bank charges a hidden currency exchange fee (my US credit cards do... 0.03%.. my US and Russian debit cards don't)
Also, make sure the cashiers at points of sale charge you in Euros. Some points of sale (La Sirena stores in Playa den Bossa, for example) have an option to put the charge through in your local currency. All they're doing, however, is giving you their bank's inflated exchange rate.