I have not had a bad one, a few small issues easily solved under the warranty or a solution found on this and other sites from members who always help.
I had an '01 and solved the injector issue with a Cat Filter first out of the tank, then on to the OEM filter which was not a good filter and allowed a lot of junk through to the Injectors before I added the CAT filter. Traded it in on an '07 which was a great truck, no issues at all. Traded that in on an '12 and had a couple sensors replaced. Traded that in on a '17 and only issue was leaking OnStar antenna on the cab. That truck got 22MPH when not towing with cruise on set at 70MPH. Maybe the best truck of all. Just traded that in on a '21. Arriving this week, maybe today and can't wait to run that 10-speed Allison through the gears.
I do a lot of towing going to car shows all over the country in the summer months (I am retired) and the Duramax has never failed me, not even a flat tire (knock on wood).
If you wonder why I trade them in every 4-7 years is because these trucks are in high demand and really hold their value like nothing I have ever seen in my lifetime. It makes it easy to trade up when you get so much value on a trade in. I just traded my 2017 Duramax with 49K and got $53K trade in value and walked away with a fully loaded 2021 LTZ, tax, license and 10-year extended warranty and rebates for $21K out of my pocket. You just can't beat the Duramax for holding value. I could have kept the 2017 another few years, but why wait at these values.
Realize this is an older thread as I was looking for info on the '21, but wanted to share my reason for loving these trucks and yes I have not had the nightmares others have suffered or my opinion might be different. Overall I don't think you can go wrong with the Duramax.
I have not seen this posted anywhere before so I will share it for those looking for a used Duramax. You can get a good idea how the truck you are considering was used by dividing the engine hours into the mileage and get an average speed of the truck over it's life time. Low average speed may indicate the truck was left idling or used around town as a daily driver.
High average speed indicates a truck that was used on the highways on trips where it maintained higher speeds. My 2017 for example was used mainly in the summer months on road trips and was not a daily driver in the winter months. The average speed when I traded it in was 44MPH.
Jus another tool for you to evaluate the truck you are considering.
I'll let you know about this '21 once I get a few miles on it.
Best Regards,