I've been to the site, and I think I'll be ordering one of their shirts within the next month, to test the quality of construction. I don't have an American Express card, but I'm told they accept direct bank transfers. Another message board - for working professionals in Hong Kong - brought me to the site. Apparently, Jantzen are a full-service tailor (suits, dinner suits, topcoats) as well as a shirtmaker, and the quality is very good. I wouldn't be surprised - I think the vast number of competitors bring out the best in most Hong Kong tailors. The only ones you need to watch out for are the ones that promise suits in 24 hours, or the ones that offer a package of 4 suits, 4 shirts, an overcoat and 10 ties at one low, low price.
Like Mike C, once a finished product is in my hands, I'll let everyone know if it lives up to the hype. Cheers, Nick.