On my 2005 F-350 DRW I have used the Timbren rubber springs. They work quite well. However, if one goes over something that causes a hard hit, then the springs seem to "hit bottom" or, in other words, one feels a hard hit.
While I like them, I might go back to air bags/springs. My recollection, though fuzzy, is the air bags might have ridden a bit better.
2011 Ford 6.7 Powerstroke F350 Super Cab, AT, DRW, Lariat. with Ride-Rite Firestone Air Bags. Used to tow 2011 Mobile Suites 36RSSB3 #5568, which is equipped with Center Point Suspension, Trailair Hitch, Level-Up, dual pane windows, Satellite Finder, 17.5 Tires, Dual Air, Valley/Husky 24,000 5th wheel hitch.