I've had great experience with the
Throttle Meister manual cruise controls. I've also tried the
Cramp Buster, which isn't a true throttle lock, it just makes it easier to relax your throttle hand.
I used the Throttle Meister on my old Ninja 650R on multiple all-in-one day trips from Omaha, Nebraska to Sturgis, South Dakota (approximate 8 hour ride). It worked great! I was able to stretch out my throttle hand and alternate holding the bike with either my left or right hand. The only down side is if you are in mountianous, or hilly areas on the interstate, you'll easily be going 100+ down the hills and 50ish up the hills. So it's really recommended to set it only on flat terrain. Same will be said for all manual throttle locks.
I used my cramp buster while delivering my sisters Buell XB9s from Omaha, Nebraska to Portland, Oregon after she moved. Let me tell you, I wish I had installed a throttle lock on the bike before the journey. Riding a 16 hour (1,000 mile) straight day, and two 8 hour days on what I've come to call "The Paint-shaker" caused me to lose feeling in my thumb and index finger on my throttle hand for nearly 3 months. So, there's that...
I have no other personal experience with using other cruise control or similar methods.