Sure the 5.3/6.0 are great motors, but I'm not terribly fond of the truck that's wrapped around them, especially for the prices they go for (at least around here).
Used two different 2000ish nbs 1500s on two different jobs, one was a 4.3/4x2 the other was a 5.3/4x4. Not particularly impressed with either of them. For lack of a better word, they seemed kind of "flimsy". I didn't see any major advantage over an obs.
Just a truck, when they get old they have problems. I see '99s that are just as beat up and wore out as any '98 on the road, yet people want them because they're newer. Fine, take it and I'll happily drive my old "piece of junk" I work and drive every day. Personally I'd take a brand new '88 over a '12, too.
Ok, the 6.0/6-speed 2500HD is a pretty sweet combo. The Duramax is another reason to go NBS, of course. Personally though I'd go with a Dodge/Cummins if I really wanted to fork out money for a diesel (I know, I know...almost as bad as a f*rd)
Just my 2 cents. If you're buying something brand new, well yes it should be more reliable than an old truck, but personally I don't want to pay big bucks for a firewood hauler. The gas mileage argument is kind of funny, swapping trucks to gain like 3 mpg......to each their own I guess. If I want mileage I'll buy a Civic, thanks.
EDIT: re-read my post and noticed I said "personally" 3 times. That should really confirm that everybody is different and has their own opinions. I'd be interested to see some more nbs people post why they went to a nbs. Not challenging that decision, just curious.