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.