Rusty has changed his spring supplier a few times over the years. I had 4.5" lift leaf springs about five years ago. Initially they went in higher than 4.5" on stock shackles but settled to 4.5" alround in a couple of months. Then during the next six months one side steadily sagged another inch and became noticeably uneven. Rusty was quick to react and sent me some replacement leaves. I rebuilt the spring pack and they were both fine for another six months then they both started sagging below 4". I used longer shackles and they were fine for a while but then started to sag again. As I was running events or doing trail recce's almost every weekend I put it down to carrying a truck full of recovery gear permanently and wheeling too much! They just plain wore out. I had some military wrap springs made locally, but they are very agricultural. They haven't sagged, but they don't flex anywhere near as good as Rusty's. I do miss the flex.
In recent years, some friends have bought Rusty's springs. Some love them, some have had sagging problems. Almost all that I can remember that have had problems seem to have been used in XJ's that regularly carried three adult passengers and recovery gear when offroad. I don't know anyone personally that has bought Rusty's springs in the last two years so quality may have improved. The XJ's SOA design really puts the leafspring under a lot of stress, I've seen almost every brand of popular spring sag on club member's XJs, even Old Man Emu HD's 2" lift.
My best ever non sagging springs were two years of wheeling on an already sagged set of factory springs with Rusty's full length add-a-leafs. I even sold them on to someone afterwards who was impressed by their sag resistance.
In my opinion, it's a component with a limited life span just like a tire and how you use your Jeep factors a lot when it comes to the serviceable life of the component.
