It is not quite unrestricted but if you enter HKG 3-4 times in a year (I dont remember which) you can register for e-channel. We did that one year and the process was really simple, about 10 minutes at the airport between immigration and baggage. When we got new (US) passports we brought the old, and they re-registered the new passports. We use the same gates at HKG residents for arrival and departure.