I Have the Jacobs/Mopar brake (the delay is there by the way as to not kick on between shifts), and it has performed flawlessly. Some of the things I like about it is that it can be used to warm up the truck faster on cold days and will kick off once the engine reaches a desired temp.
The drawback to this is my cab smells like diesel whenever I do this. May need to be retorqued however.
Also if you have the switch in the on position while the cruise control is set and lets say you are going down a hill and gain too much speed, it will kick on automatically to slow you down.
It also has a port for an EGT gauge, a great warranty and I like the fact the Cummins clearly says this brake was in mind when the commonrail was designed, so its not an afterthought.
However If i could do it again, I would go with a different brake. In my case (03 truck, 3. 5" elbow) having the jacobs brake (or any other turbo mounted brake for that matter) totally limited my choice on aftermarket turbos. Twins are out of the question and my choice in singles was dramtically reduced. If I could do it over again, I would probably get an inline 5" pacbrake so I could of gotten twins. Just do your research and decide what your goals are before you make my mistake.
Dave