When I went to UK for 5 weeks I just bought local SIM with data, and just stuck in phone. No idea if is unlocked or not but worked fine (Galaxy S3) there were loads of options for data etc. HK will be similar, you can prob find out best option on net in advance. I wouldn't mess with global roaming myself, too many horror stories, Also you don't have control if someone phones you then you are charged the extra and they are only charged the cost as if you were in home country. As you can't control who will call you then if your SIM isn't in the phone then there is no way you can be charged.