They tend to create a hard ride and can break springs. That being said I have done them front and rear plus a shackle lift for a quick pick me up for larger tires.
x2
I had a similar problem with my fronts being negative arched when I bought my truck. I added the add-a-leaf to the front as a temp fix until I can afford my Marlin springs. It definitely makes for a rough ride on the street and the flex is hindered as well.
