I’m not a big fan of exchanging currency because it requires you to carry large sums of cash while traveling, which is never a safe thing to do. Using a credit or debit card to get cash from the ATMs in Paris usually gets you a better rate anyway. But if you can’t avoid exchanging cash, at least try and get the best rate possible within Paris. Usually to do this you’d have to call around or check a bunch of websites, but an enterprising young French techie with some free time decided to create a website that allows you to compare rates all in one spot, Compare Money Changers. He doesn’t work for these agencies and doesn’t make any money off the site (for now), so consider it an unbiased service, and always confirm the rate with the change office before handing over your cash.