i have found that depending on the spring, if the shackle angle is near vertical, it will ride like ****, with some of the leaf packs my buddy's have made this has been the case.
My personal SAS, i have a 15 degree or so shackle angle at ride height, using generic rancho 5000 shocks rides almost as good as my IFS did, and lots smoother than most of my buddy's straight axles.
As zukkev said, it does depend a lot on the leaf pack, generally, the flatter the leaf pack, the smoother it rides, the more arch it has, the rougher it will ride, also, depends on how many springs are in the spring pack AKA (as zukkev said) spring rate