I own an 05' (9-04' build) 350 cc lwb 3.73 Lariat. Tow the 26 TH, loaded down.
I also own and operate an auto repair facility. For what it's worth...
I can tell you that there's not another "stock" power plant on the market that can keep up with the 6.0. However, with a few modifications - there's several that can run side by side.
We're talking the whole scheme of towing. Not just fuel mileage. IE: Mileage, 0-60, 40-70 (normal interstate entry speed) and the oh so ever over looked stopping ability.
Being able to get the rig moving is one thing and being able to stop it safely is another. There's many times I've had to get down hard on the anchors and never really felt out of controll. I average 15-19 city (pending my driving habbits. The 6.0 runs like no other diesel and the right foot gets a little heavy at times) and 9-13 towing. Again, pending load and conditions. The best story I have is I recently took a trip to an ATV park in Madison FL. The park had just recieved 12" of rain and the main drive to the camping site was axle deep the whole way. I was one of only a handfull of trucks that didn't require assistance to get through. Only one of them was not Ford and that truck had more in modifications than both of my ATV's cost. (Cummings)
On to a little more specifics about the Ford PSD. Synthetics are your friend. They're a little more up front in cost, but average out over the long run. Filter selection and quality of lubricants is critical. Chosing a run of the mill filter will shorten the life span of any vehicle, even more so in the 6.0.
The 6/0 wants to work. It "needs" to work. IF this will be more of a daily driver (grocery getter) then perhaps this isin't the truck for you. This truck needs to reach optimum operating temperatures to burn the "coking" of the turbo veins and let the EGR get warm enough to burn unused fuel as its designed to do. It don't like to idle for lengthy periods because of wet stacking (unburnt fuel sitting on top of the piston) and causing build up over time and eventually blow by. Hence my mileage being all over the place. I run a tank of fuel out at higher temps. My truck is nearing 70k miles and I've not had one issue. I service it at 12.5k miles using only AMSOIL products.
After mid 04' IH worked out just about all of the bugs that were thrusted on them in 02' when Ford approached them about finding a replacement for the 7.3 (which was all but bullet proof). All of the Turbo and EGR issues have been ironed out and there's only a small number of problematic trucks left. Which will be in any make.
The new 08+ Cummings motor has many of the same features that the 6.0 has had all along and some new ones as well (built in jake brake), etc...
The best advice I can give from a technician stand point is to ask for (or request) an OASIS report on the VIN# you're considering. It'll ive you a history of warranty work done. Which, by the way, Ford is offering a 2yr 200k mile extension warranty through an ESP plan. Just see a local dealer. Take a look at the report and see if there's been a rash of repairs. If so, then perhaps look elsewhere. However, reaching 50-60k miles and minor issues is proving to be the norm for getting a good truck.
Here's a link to a forum I frequent. Alot of knowledgeable and honest folks here.