I moved up to a 6.7 diesel from a 6.2 gas. I'm very happy with the change. My 6.2L would need to be nursed to get 15mpg on the highway at 100km/h, the 6.7L has gotten 22mpg on the highway at 100km/h a few times. There will be naysayers and "haters" (I dislike that word), but I hand calculate each and every tank using the imperial measurements and this is what it is. Since buying the diesel this past December, my lowest recorded consumption was 13mpg. You will not be disappointed in the change...
As for taking a long time to recoupe the added costs of buying a diesel, you'll break even in 3 years or so as I've figured. Also in my research, the diesel depreciation is usually less than that on gas vehicles...
I'm not taking away from the 6.2L show at all, it does exactly what its intended to also!!!
1990 F250 5.8L 4x4