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.