I had D25's on my 2001. 5 and like Scrappy, they began to sag 1. 5" (after 12 months).
I have the KORE Recon suspension (I know, different spring than HD Leveling) on my 2005 and couldn't be more happy with my purchase. KORE provides a lifetime warranty on their HD leveling kit's shocks and springs, so that in addition to the fine engineering of their suspension systems and their outstanding customer service should be a no-brainer. A bit spendy, but you get what you pay for (my $. 02 anyway).