Ken,
I have them on both my bikes. As Bob mentioned; when touring I'll use it to lessen the return pressure on the throttle grip to limit hand/arm fatigue. It's a simple thumb screw with spring that applies friction to the throttle grip tube. I call it a thumb screw but never been able to operate it with just the thumb, it'll take a thumb and finger to operate. I normally set it where I want it before venturing on the bike since it's a little cumbersome to deal with on the fly. Not sure if you could really call it a cruise control but maybe a cruise aid. I wouldn't recommend turning it to the point of locking the throttle - since it may become a safety factor. It screws into a threaded bore at the bottom of your perch most likely covered with a clear plastic plug. Just remove the plug and screw the device in to the desired tension.
I've become attached to them on both my bikes.