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.
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.