I just traded my 2005 Ram 2500 QC 4WD SB ST w/ HEMI+M5 to a 2006 QC 4WD SB SLT w/ Cummins+A4. The HEMI + M5 tranny were OK, but a bit weak, and hard on gas at 11mpg (13 on a very good day). The M5 wasn't particularly good at in-town driving. Highway driving wasn't any better because of too much gear. The Cummins + A4 is a different story. When empty, the truck is quite sporty and the A4 has pretty decent road manners. I can't wait to put it to the towing test. Fit and finish are very good on the '06. Road and engine noise are quite acceptable. Overall, a very good, powerful, truck for the money.
Massive amount of torque. Improved fuel economy over HEMI. Better looking dash (but see below). Very good ride for a 2500.
Burled wood interior looks cheap. Power vertical sliding rear window. 3.42 rear gears for better mileage. Grill seems a bit more flimsy than '05. Needs a shield over top of AC condensor - ouch! MP3 CD capability.