My 06 was built in 8/05, I found and bought it 2/06, it had 22 miles on it. I'm at 16,500 miles so far with about 25% towing.
Warranty items:
Clutch master/slave assembly replaced
Steering box replaced
long crank TSB flashed (fixed it for a while, still does it accasionally)
Fuel tank roll over valve recall fixed
A/C blower motor issue just broke last week, has not been fixed yet
Strange interior light issue that fixed itself
Cruise control has cancled itself twice for no apparent reason
Mileage:
Empty with minimal idle time, 17-19mpg hand calculated both city & highway as long as I run 65mph or less and shift around 2,100 rpm. I saw 19 once on one tank, most the time its 17. 75 to 18. 25 always hand calced. Long idle times will drop numbers fast.
Towing 38' 5th wheel toy box, GCVW 23k+ at 65mph 90% highway, +/- 9mpg hand calculated
I love the G56, it's awesome for towing! The pleather seats are not nearly as comfy as my 04' SLT seat's. The A/C seemed to work better in my 04' too. The headlights on this truck are better and I love the electric rear window and sunroof.
I do all my own maintenance. I cannot believe the design for lubing the front drive shaft actually made it to the trucks, what a PITA. My 04' never had any problems with the front drive shaft nor did anyone I knew. I really hope a better design comes to the surface.
I'm satisfied overall.