It is no wonder GMC's cannot compete with Ford any longer. I was sent an email to update this -12/29/17. Not sure why! Still the same title for sure but now worse, entire dashboard is not functional, speedo does not work, all other gages peg out when starting the truck. At least it is running-mostly. I have to hand pump the diesel filter to get the truck to start. Happens about once a month or so. (no it is not standar maintenance issue). Replacing the batteries annually now (seems something is draining them when I am not looking!) I have received a call from Cust support about the dealer charge offering to reimburse some payment. I would have even considered changing my review if he would have helped. Then he disappeared! From the Dealer using my truck to tow the owners horse trailer (when turned in for maintenance) and adding 100 miles to it...to the more than $3000 I had to put into the electrical within 3 months-to the instrument panel stopping to work (speedo, then oil pressure, then battery-which comes and goes) $900 fix it so I just use a GPS for my speedo-to the rattle in the steering because of design issue always going out....to the electric seat positioning clicking 7-8 times before settling into place-to now the heater/AC blower stopped working and a cost of $750 to fix it at PEP Boys. Thanks to youtube the heater fix will only cost me $150 in parts. (my friend just had the same thing happen to his Tahoe and had to pay $750) The heater went out again in a couple of weeks. replace resistor again, heater stopped again! Either this truck is a lemon OR my favorite GMC's are on their way out of business. (I am in Quality management and would not have allowed anything close to this happening where I work)
I purchased the Sierra dually Duramax (LLY - 310hp & 605flbs) to pull a heavy fifth wheel and so far my experience has been trouble free. So far I am fully satisfied although I only have 30,000 km and time will provide more information on reliability of the Duramax. I find it fun to drive and fuel efficiency is good compared to the other diesel (Ford & Chrysler). It provide plenty of performance compare to a gas engine. The interior is perhaps blend as per the critics but it provides a price advantage compare to the Ford's King Ranch. It does not take away from the performance.
I think that the Allison transmission is the main feature of the truck. The exterior design around the dual wheels has a more aerodynamic appeal.
The interior design could be improved but not at the expense of reliability. There are after market products available to increase output (hp & torque) that I wish GM would test and permit without invalidating the warranty.