I've got an '04 2500 Hemi and hate the auto transmission, well not the tranny but the way the PCM controls the shifts. Dodge really screwed up on this one. There is no way of controlling O/D, it only has a tow/haul button. In tow/haul it locks up the torque sooner which is fine for hauling but then it still allows it to go into (the first) O/D, so when it does it falls on it's face. Then going downhill in tow/haul it will decide when it wants to drop from O/D to drive and if I am on the gas a little going down certain mountain passes it upshifts when I don't want it to. A diesel would probably be alright, enough torque to run around in O/D but not the Hemi - this motor has no torque below about 2800rpm.
Just drove an '08 Cummins with the new 6 speed auto, what a treat! They finally put some driver control into it with a small rocker switch on the shifter which allows you to control all 6 speeds, sweet!
If I had it my way, I would own a '03 - '05 Cummins with a 6 speed manual but the wife can't drive a standard which would leave me being the designated driver all the time.