Hey there! good to have ya around here! I must admit that I was sceptical about owning a dually as well, however now that I have one, I would not be without it! But, you have to consider that i do haul a lot, tons of weight and lots of miles. I notice that my truck might get 1-2 MPG less than the others, but that could be because I have this little tendancey to keep my foot in it at all times :p (I just can not help myself!!!)
Over all, if you are looking for a truck that can do some off-roading, be sure that you get the 4 by 4 option as a 2 wheel drive dually (MINE) is pretty much helpless off road. But I must admit that it is better than epected for a 2 wheel drive, so the 4 by 4 dualls aught to be really good.
Either, way, if you are not really hauling anything huge and GN, then you really dont need the duals, just remeber that when you go to buy the tires, rotate them or put nice rims on it, you are looking at WAY more cost, and usually less selection.
Hope this helps!