You pay commission (on sterling TCs, usually small) or suffer an exchange rate loss (with $US TCs, often around 5%) when purchasing them
You may or may not have to pay a charge when changing them overseas.
If you have them left over you can cash them (usually FOC with sterling ones) at home. With US ones you'll suffer a further exchange rate loss - more likely nearer 8% than 5.
In general I'd avoid $US ones - you pay two lots of charges - unless there are very particular reaons for having them where you are going.