My wife's truck has just short of 60K on it's 6.0, no issues. Just a little csk.
We bought her truck with 54K on it, however, I know the previous owner and his only complaints were replacing the A/C switch on the A/C receiver dryer twice.
I have towed my gooseneck trailer with a Bobcat on it behind her truck, approx 10,500lbs, and it motored on like a champ.
We probably will have this rig for a long time.
My wife's dad however, has a GMC with the 4.8L engine. Essentially the same engine as the 6.0L, and his truck is approaching well over 150,000mi.
GM hit a homerun with the 6.0L engine, it is a shame they struck out in their next at-bat by discontinuing the 8.1L engine.