Gimmick and "dangerous". I've read many 2nd/3rd hand reports of them doing weird unloading while wheeling.
If you want something a little more predictable and smoother than shackles, look into Liquid Iron Industry leaf spring sliders:
***** Current Lead Time is 2-3 Weeks Until Stock Is Rebuilt*****The Liquid Iron Industries Universal Slider Box Kits are designed to significantly stabilize long travel leaf spring suspensions on practically any vehicle. These slider boxes completely replace the standard shackle configuration...
liquidironindustries.com
Onroad is a smooth silky ride going faster than you are willing... I won't claim having seen much more than the speed limit using them in my FJ55. With a sway bar in the rear, it handles backroads like no tomorrow. Without the swap bar, poop your pants if you hit a pothole doing more than 55mph
Flex is pretty damn amazing:
View attachment 2465881
Flex stopped because of front fender and drag link bind. The above is also with a sway bar in the rear. (60" wide Dana 60 axles + 54" Tahoe springs)
I'm doing the same in the rear of my 3rd pig... and when time allows, my FJ40 will get same treatment.
Here is a "slow" ride of the above after not driving it for a year on a damp road(plenty of speed left):