From the Dealer using my truck to tow the owners horse trailer 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 diesel itself is great, when running.
I love the size, the big interior, the carrying power.
The reliability and the response from GMC are the only problems.
(if I received the support I would expect, I would be #1 supporter of GMC)
I do not/cannot trust any of the dealers and the customer service person in Detroit (or where ever they reside) "almost" helped me but ended up not helping.
I will never buy another GMC as long as I live and will tell all that I know of the problems that I have seen with the company.
I believe there should be more monitoring of what goes on at their dealerships, better selection process or something.
And, stand by their design flaws.
Proactively, not reacting to a crisis before something is done.
(cannot be a coincident that my friends and my heaters went out at the same time.
(PEP Boys told him that happens "a lot" with these.
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.