I'm in the same boat as you Traffic.
I do recall reading in these forums about a year people having problems with coverage from 3 in HK. It's a bit hard to search for "3" or "three" in the forums though! However, to further that, our office manager cannot get coverage from 3 at home... she lives in the NT but I cannot recall where.
I've found Smartones' coverage to be superb, both in HK and abroad. I travel quite a bit so roaming coverage has been important. However, we have a corporate account with Smartone and that service has been horrendous. I'm lucky in that we've been switching our service over to 3, so my number will be transferred on July 11th
One plus about 3 -- in the year since I recall seeing the threads about 3 here, I seem to recall hearing that they vastly improved their roaming fees to other countries where 3 operates.
If anyone has experience with 3 abroad, I'd be curious to know that too.