I've had my 24 Sierra SLT with the LZ0 for a year now and surprising already have 19K miles. They have been all trouble free and the truck has been flawless. I came out of a 2017 Yukon Denali the the 3.6 liter and it was a great truck. I put 112K on it before we sold it to our daughter and her husband, and other than a transmission issue around 40K miles, fixed with a full flush and refill, it was flawless. I think he's got nearly 140K on it now and still running strong.
Back to the LZ0 on my Sierra, I average 28 mpg most to the time. Diesel in my area runs $3.09 a gallon so it's been very economical to run. I refill the DEF tank when it drops below 1/2 and I'm filling it every 2.5K-3K miles. Oil changes are a breeze too with the drain plug and filter right there. I expect I'll need to replace the fuel filter in a few months. Most of our miles are highway between our home and our farm about 50 miles away, but it's also become the go-to for longer distance trips instead of my wife's Audi Q5. I absolutely love this truck. It's a standard bed with Max Tow so it's also a workhorse when I need to tow or haul something. I strongly considered getting a 3/4 or 1 ton with the 6.6 gasser but the cost of entry vs my 1/2 ton and the expected fuel expense at 12-15 mpg were just to hard to swallow. I may still need to jump up to the 2500 or 3500 at some point if i end up maxing out my tow capacity on the 1500, but I'm pretty happy now and hope I won't have to trade up in size.