Got a '06 5.9 3500, also with the 6 sp manual, put almost 10K on it so far. A good combination of power, comfort, and mileage. If I keep it @ 65/2100 rpm it will get 23 mph on a good day.
One issue is the injectors need replacing @ 130K. Not cheap.
Eagles may soar, but a weasel never got sucked into a jet turbine!