Don't use one. It's crazy expensive.
You'll pay whatever the lowest exchange rate was within 48 hours, plus charge you a percentage.
AmEx isn't too bad, and I've argued with them extensively once. I got my money back. I've had to use it a few more times, and the rate they used was right on. Maybe they somehow flagged my account that I'm a tightwad and watch.
But here's what I do -
Go online, find out what the day's exchange rate is. A good source is yahoo finance. Type in the abreviation for the money to exchange, and then scroll to the US exchange.
Go to a small town and find a currency exchange. Barter. And double check the math.
Don't use an airport exchange or a bank. They are way off.