None at all from a UK viewpoint - used 'em in the States (DC, NY, Florida), Spain, Portugal, and even, ahem, France. The pesky petrol pump readers in France don't like UK cards much though. Also, the ATMs in Spain are sometimes inside the bank door, and swiping the card won't always open the door!

We've always taken an emergency float of travellers cheques as occasionally in-store card readers don't like UK cards, again especially in France.

Be warned that the system (as always)takes its cut - if you're sensitive to exchange rates, don't look at your bank statement too carefully. The convenience outweighs this though IMHO.