x2 to what John said. Delphi spent 10 years developing the rear-steer for these trucks, and they were subjected to pretty rigorous off-pavement beatings to ensure the quality of the components and design. As far mileage goes, my truck has the Hypertech MaxEnergy tune, K&N and muffler delete and I get 19 mpg @ 65-70 mph. Average is 15.2 mpg right now with mostly city driving and a cautious foot. I have the 5.3l.
The only problem I've run into is finding aftermarket support for lifts. The only kits that say "Quadrasteer compatible" are 6" lifts, and I don't want to run 35" tires and average 11 mpg. I have a sneaking suspicion that most kits will work since the spring mounts are the same as non-4ws (afaik). If anyone knows otherwise, please correct me.
As a daily driver, I highly recommend a Quadrasteer truck. I live in a large suburban area with lots of tight turns and small parking lots and spaces and have never had to make a 3-point turn. It's safer, negligible additional fuel consumption, improves tire wear (so I've read) and turns heads.