I work at a shop that has installed and removed a few of them due to repeated failures. If you haul, they do not hold up to deceleration. basically they are not designed to take a load on the backside of the gears.
I know plenty of people with them and as was mentioned if you are engine braking then you must put the gear vendor in 1:1 not the OD gear. Oh and you have to change the fluid about as frequently as you change your engine oil. I have found very few people who will tell you that there was an economy increase of any consequence.
I should have clarified my explination a bit more. BJS is right. I was referring to engine braking being hard on em'. One truck we pulled a gear vender out of, hauled a heavy trailer a lot, and had an exhaust brake. Maybe if you don't haul often, and don't have an exhaust brake it may work for you.