I have them and use them when I'm hauling my motorcycle or other stuff.
I went cheap and did not get the air compressor. They require from 10 to
100 PSI but the volume of air to fill them is small. I got a "slime" brand
12v tire inflator from WalMart for less than $20. It easily and quickly fills
them when I need to. This is typically once before I leave home and maybe
once more if I let the air out of them to help lower the rear to facilitate
unloading my motorcycle.
In my case they help with keeping the tail level so my headlights don't
shine up in the trees and other folks rear view mirrors.
Also once in a great while I haul something heavy.
I once went and got bulk mulch. They heaped my bed full. I drove home
with a level truck. Others that were leaving the mulch pile looked like they
were ready to drag their rear bumpers!
The compressor is up to you. If you want to save money you can do
as I did and manually fill them. You could even fill them with a high pressure
bicycle pump like they use for those skinny tire bikes. It's not like filling
a truck tire. It only takes a little air to fill the air bags.
I can't really say they helped the porpoising much. They just get me level
when I've got extra load in the back. We sometimes haul a 500 LB motorcycle
and firewood and tow at the same time.