I've seen them before, but never needed them myself. I've towed a lot of miles with a gooseneck half ton, three quarter ton, and one ton, and never seen the need. I haven't towed beyond a 30' trailer though, nor over 14000lbs.
The only time I've had issues with a half ton tagalong trailer was when I was driving across New York on a very poorly made concrete highway. When the truck was climbing a small uphill concrete pour, the trailer was going downhill the previous one. The entire 100 mile drive was constant see-saw. Only time I ever got carsick while driving. I can't imagine what the poor horse was going through. I don't know if this would have helped, but it certainly wouldn't have hurt.
To me, with my load, I've found the best thing to add is helper airbag suspension. It really does a great job of softening the rear suspension on the truck. Every truck I've owned from half ton to one ton has had airbags installed (they're about $250, and take two or three hours to install in your garage).
What kind of trailer do you have? And what truck do you tow with? What concerns to you have that suggested that you look into this product?