Recently bought my 2005 ext-cab Z71 silverado 1500 a few months ago, used with 90,000 miles on it. I'm the second owner. Good things: Engine is still very quiet and runs smooth for having 90K on it, great on gas, ride is smooth, 4wd still works, interior in great shape, no electrical or steering problems. Overall still a great truck inside and out. Bad things: Doors rattle a little when driving over rough bumps, transmission shifts hard into 3rd gear and makes a "smack" sound. A/C makes weird gurgle sounds after I turn it off.
Z71 package, rides smooth for a 4wd, wears the bowtie, A/C is ice cold, good on gas, very comfy seats, very stylish lines.
Give it a smoother shifting transmission and improve the firmness of the doors on the ext cab versions.