I have a kelderman on my 99, I got the 7-9 inch lift version. I love it, but it hasn't been totally trouble free, but I have to say all the problems are my fault, too.
I put got the manual kit, not the fancy one with the electronic ride height, and I put the solenoids under the bed, back by the axle. BIG mistake if you live where it goes below freezing, they get ice in them and don't work, or stick open and it goes up or drops as I drive down the road. Most of the problem is that the tank they provide does not have a drain fitting, just one port in each end, so there's no way to drain it. I ended up mounting it vertically in the bed (temporarily) so I can drain it once in a while. I also had to replace the pressure switch, it was under the bad, and the contacts corroded due to the mag chloride on the roads.
My plan, (when I get time) is to put on a commercial truck ride height valve, with a dump switch. I got one for a freightliner, with the bracket kit, it looks like a piece of cake to mount with almost no custom fabrication, as soon as I figure out how which hose goes where on it.
The kit itself was great, everything fit great, the parts are massive, ride is awesome. I've had 4000lbs of sand in the back, came off the scale at 13,900lbs. I think that was close to a max load, I was at 120 psi in the bags, but it rode level. They told me their kit is more for ride quality than to increase capacity, and you'll still be limited by tire capacity, wheel bearings and brakes from going much over that.
All in all, it was one of the best things I've done to the truck
99 F350 CC DRW 4x4 PSD 6 spd 4:10 Programmed, 4"exh, 8" lift Kelderman 4 link air ride rear, southbend clutch (and arm!) 295/85R22.5's, AFE stgII gauges, push bar, 07 lights, recon smoked fender and rears, ARE shell, DynaTrak combo front,
, Short throw Shifter, Airdog, Regulated Return fuel, Coolant filter, Bypass Oil Filter. Next:BB Turbo, CAC, Injectors, HPOP, Head studs, and anything else I can afford