I've only used them once, and the only mistake was mine; I forgot to uncheck the "next day" box!
Cost me $100 (plus about $60 import duty 'cos the good ol' British government charge it on the goods AND the shipping) but I was so impressed with the service that I hadn't the heart to complain!
Simon and Garfunkel were right, though; you've got to keep the customer satisfied!
