I live in the UK (always have) but "joined" starbucks rewards back in 2005 whilst in the US ... and use the card here always no problems. Some of the offers - ie the free breakfast don't work but the free coffees do - also the US card is better than the UK as think it is only 30? stars for Gold whereas UK (unless changed) is 50 also 10 instead of 15 stars for free beverage.
One thing I also find the card useful - if I am travelling to a country I am unlikely to return to I just find a starbucks and load my card with the loose change/currency.