Charles, I will put it this way: I have a 2002 Envoy with the load leveling rear, its great. I love it... except when it $*@!es up.
I have 119,500 miles and have replaced the compressor thrice. I had to replace the switch in the trunk to turn on the auxillary (air hose to inflate tires, etc) a few months ago as it went kaput.
Its a maintenance free system - no cleaning, oiling, or other cleaning is required. But when it poops out, you are going to hate life. I bought 2 compressors on ebay (brand new) for $250. The third one I bought used for $20 or so.
My advice: Keep a spare on hand, because when they go, they are hard to find and take a few days to acquire. The dealerships usually want $700 or so just for the pump.