I have one in the rear axle of my F350. (Dana 80) Well worth the money in my opinion. 90% of the time the truck is on the highway, or at least pavement, but the 10% of the time it is off road, especially with a heavy gooseneck behind it, its been a god send. Fully controlable, positive engagement when I want it.
I was gonna go with a Truetrac until I read some horror stories about them blowing apart when turning sharply on pavement when loaded HEAVY. Not wanting to be stuck on the side of the road with a blown diff, I decided to spend a little extra cash and get the ARB setup. Its been flawless in operation, and really worked well time after time. Half the time I just use it instead of getting out to lock my hubs in for 4wd and even in 2wd the ol dually just keeps pullin through.
Randy's R&P got me a real good deal on a new set of gears and the install kit, and a screamin deal on the ARB itself. Even though its a bit pricier, and the installation is more involved, its been well worth the money.