I don't have any complaints with mine...............stiff but a little time made them relax. my favorite part is I can buy one spring in case some lunatic rearends me and jacks up my leaf.................
I don't have them on mine-yet-but picked them for mine.My buddy has them on his '71 and loves them. He had Black Diamond and they were so D*#m stiff you didn't need to go to the dentist because driving the thing would rattle any loose teeth right out (very stiff and poppy) and we had done our part to work them in before he threw in the towel. The Downey gives good flex for a weekend wheeler and also rides good on the road-plus you can't beat the price when compared to some other kits.
I have offered the same leaf springs as Downey for over 8 years now, and finally got around to installing a set on my own FJ40 last year. I found them to be super-flexy, so much so that they scared me a little at first compared to the MAF heavy-duty springs I had on before that.
I'm running Downey 4" springs and their 1.5" over stock rubicon shackles. They have perfect shackle angle and a relatively soft, flexy ride. My only complaint is that they are slightly narrower than stock springs and I had to search around for awhile to get the proper bushings to get the shackle sideplates parallel.
I ran stock length shackle in front. I also had to screw around with bushings to get things lined up. I then added 4 degree shims after measuring caster at the alignment shop and coming up with ~ negative 1 degree.