Experienced them in LAX and HKG

LAX: Pretty useless, since they cannot put you thru immigration any faster. I actually would recommend not using them but to walk to immigration as quickly as you can - otherwise you will end up behind other passengers

HKG: They take you to a special immigration booth, disappear with your passport and then let you use the handicapped line. This can save some time if immigration has long lines, otherwise this can add a minute or two.