I've got the Tuff Country 3.5" lift on my YJ. It sure wouldn't have been my choice, but the previous owner did choose it so I've been wheeling it for almost a year now.
I haven't spent much time in other YJ's recently, but it seems to be pretty stiff and bouncy. It didn't have any flex when I bought it, but that could have more to do with the tires running out of room and the bolts on the stock shackles being torqued way too tight. I have since installed some Rough Country 1.25" boomerang shackles at what seems to be a very reasonable torque level, and my flex has improved considerably. I didn't notice the ride getting any better, though it probably did.
I will say that I have beat on the springs a bit with no issues. I even snapped a stock shackle in half while down in Moab and couldn't find a replacement, so I drove home (about 240 miles) with only one end of the front right spring mounted to the frame. Honestly...other than a little bit of a lean, I really didn't notice it. I'm guessing that has something to do with the stiffness of the springs, but I am very pleased with the abuse my springs have shrugged off.
If I were buying a new kit today, I'd go for the RE 4.5" Extreme Duty, and I may still upgrade to those springs or something comparable in the future, but for now my Tuff Country springs (and shocks) are doing pretty well.
'90 YJ - 2.5, AX-5, 231, open 30, 8.8 w/ Detroit, 3.5", 33's...
Utah Jeepcrew #26