I've driven 2 of them so far, both 2WD models since that is all that the dealers had. Both had 3.55 rears. The one I drove locally I put about 30 miles on in somewhat mixed driving, and the EVIC read 26.1 mpg (which owners are reporting to be accurate). The other I drove when I was in Albuquerque, and only put about 20 miles on. The EVIC read 24.9 mpg.
Both trucks drove beautifully! The little Ecodiesel drives like...well, a little diesel. Mid range is where it's at with this thing, and the 8-speed seems to keep it in a happy place all the time. There is a slight lag off the line, but then she picks up an moves. The power is very linear; not "throw-you-back-in-your-seat like a hemi or Ecoboost. All around driving manners were excellent, and you'd be hard pressed to even know it's a diesel from the noise level (nonexistent) and general feel.
I live at 5500 ft. elevation, and can be over 10,000 ft. in a couple of hours, with plenty of mountain passes a stones-throw away. A turbocharged diesel will have real advantages for me over an NA Hemi. I know quite a few people who own Ecoboosts, and their real world mileage, combined, seems to hover around 17 mpg on a good day. I don't consider that bad for the power of that engine, but if an EcoDiesel can crank out 24-25 combined (which is what owners are reporting on the Ecodiesel forums) then that's nothing to sneeze at, even with the increased cost of diesel fuel.
There is no doubt this engine isn't for everyone, but for those who can realize the advantages, I think it's a winner.