I tried a few of them on my Gen I's. The Throttlmeister works well, but they're much more complicated and expensive than the others. I don't really remember the brand names I tried, but they fall into categories. One kind just holds on the right grip and swings into position when you want to set it and it holds the throttle in that position, because it locks into place behind the brake lever. Another is a friction lock with a thumb lever that you flick on and off when youu're at about the speed you want (this one is my preference). These all work fine so you can grab something out of your pocket, or stretch your arm or hand for a minute, etc., but they don't hold your speed very well; you just gradually get going faster or slower because they can't adjust for outside factors. The last two kinds can be $20 or even less. Don't spend much more than that. I've some that work just like them that look all chromed up and fancy for three times the amount. All of the above types can be easily overcome by just twisting your wrist to slow down; they don't hold beyond your ability to override them. By the way, IMO the last two--cheaper ones--work exactly as well as the Throttlemeister, and they all have the same weaknesses
The last kind is beyond worthless. They sell them in bike shows and everywhere, it's just a paddle-shaped thing that wraps around the handlebar/throttle that you rest your hand on so you don't have to grip the throttle. It holds the throttle with just the weight of your hand These things are DANGEROUS! DON'T BUY ONE! There was a big discussion about them here several years ago, and a LOT of guys had tried them and ran into the same thing that happened to me. Say you're on the highway for a while and the thing is set in a certain position, then you come into a town. Unless you're very careful with the positioning of the paddle, it's just so easy to nudge the damn thing down and goose your rpm's in a VERY sudden and unexpected way. It would be so easy to rear end somebody, or shoot out into traffic, or do a surprise wheelie, or a lot of really bad shit.
Haven't seen anything about these damn things in a long time, but it's worth raising the issue again as a reminder. They're cheap and look like they might work. Don't.
Expensive but very happy with it and much cheaper than a new bike. 
