DNickisch,
I just took the U. S. Gear Decelerator off my (wrecked) 2000 truck. I don't need it on the new 2008, so I will either re-install it on my 1995 or sell it.
I was very pleased with the U. S. Gear unit. It isn't as well known as the air and vacuum operated units, but it did the job well for me. It will work on any truck with about a foot of straight exhaust pipe and a battery.
It uses electrical solenoids to control the valve. The solenoids are like the fuel shutoff solenoids, with a pull coil and a hold coil. There is a control box inside the cab and a wireless push button that signals the control box. It does require some heavy wiring (10 gauge - 25 amp), but installation is quite straightforward.
Mostly I was happy with its ability to slow the truck. I don't have any experience with the other brands, but the U. S. Gear would keep the truck and trailer below 60 mph on all but the steepest downgrades through the mountains of West Virginia and Maryland, as well as New England.
Just another brand for you to consider. From what I've read, you can't go wrong with any of them.
Good luck,
Loren