I bought an 01 Suburban with the 8.1/4L80E back in March.
Then, in April, I bought a 35' Forrest River Wildwood camp trailer.
In July, I spent 17 days pulling said 35' camper to Colorado and back with the wife, four dogs, two boys, and a household worth of stuff.
I LOVE the 8.1.
We got between 5.15-6.35 mpg, setting the cruise from 60-80 mph. It's not made for mileage. It's made to pull 12000 pounds across I70 in a timely fashion. The mountains were never an issue, and at no point did I feel as if we were asking more of the truck than we should.
Our last day in Colorado, we were leaving Montrose for Ouray to hit the hot springs. Just cruising along when she went to limp mode. Once we got through the day, we found that it was throwing a TPS code. I'll shorten the story by saying that it now has a new TPS, Throttle body, Accelerator Pedal Position Sensor, Throttle motor, pigtail at the TPS, and reflashed computer.
I love the 'burb. I love the engine. Fly-by-wire can bite my ass.
NEVER FORGET 9/11!!!!!
Hung like Einstein, smart as a horse.