Amex is generally accepted in larger stores or where tourists might shop. Having a backup payment method MC/Visa/Cash would be a good idea, though.
It's not as widely accepted as it is in the States and it costs the retailer more to accept than MC/Visa.
Also, be sure your card has a Chip in it and carry a pen. One of the side effects of Chip & Pin is that no one ever seems to have a pen around to deal with the Chip & Signature cards issued in the States.