Happy to share my recent experience with this.
I just bought a 2011 GMC Sierra 4x4 1500. About 2 months after purchase, I had a leveling kit installed on the front. It's a spacer that sits on top of the strut to lift the front up 2 inches roughly.
Truck looks great. But by lifting the front, the rear was quick to sag under load. Three dirt bikes in the back, or towing the boat, or any load really would make it sag more than I wanted. Also, under load, with the front now lifted and the rear sagging, the steering was more susceptible to over steering on the freeway. Front wheels felt light. I'm picky, most probably wouldn't care.
Step 2. I had the airbags installed with the digital controller in the cab. Now under load I can level out the truck most of the time. The system keeps 5psi in the bags at all times, so between this and the leveling kit, the ride is a little stiffer than stock when not loaded. And I probably have the equivalent of a 2" in the front, and a 1 inch in the rear.
My truck will manage a heavier load than stock, and it looks nice, but I did sacrifice a little bit of drive/ride quality when unloaded. I think the factory GM bags that come with the higher end models probably do a better job of maintaining factory ride quality.